Interference-resistant compensation for illumination devices having multiple emitter modules

ABSTRACT

A method and light emitting diode (LED) illumination device comprising multiple emitter modules are provided. In one embodiment, the method includes bringing to a level insufficient to produce illumination the respective drive currents of all except one of multiple emission LED elements within respective first and second emitter modules for the duration of a measurement interval within respective first and second series of measurement intervals. The measurement intervals are interspersed with periods of illumination, and the first and second series of measurement intervals are separated by respective first and second offsets from a timing reference. An embodiment of an illumination device includes multiple emitter modules, where each emitter module includes multiple emission LED elements and one or more photodetectors. The illumination device further includes a lamp control circuit adapted to perform steps of the method.

CONTINUING DATA

The present application is a continuation-in-part of the following: U.S.application Ser. No. 13/970,990 filed Aug. 20, 2013; U.S. applicationSer. No. 14/097,339 filed Dec. 5, 2013; and U.S. application Ser. No.14/314,530 filed Jun. 25, 2014; each of which is hereby incorporated.

BACKGROUND

1. Field of the Invention

This invention relates to illumination devices and, more particularly,to illumination devices comprising a plurality of light emitting diode(LED) elements and to interference-resistant methods for monitoring andadjusting the illumination devices during operation.

2. Description of the Relevant Art

The following descriptions and examples are provided as background onlyand are intended to reveal information that is believed to be ofpossible relevance to the present invention. No admission is necessarilyintended, or should be construed, that any of the following informationconstitutes prior art impacting the patentable character of thesubjected mater claimed herein.

Lamps and displays using LEDs (light emitting diodes) for illuminationare becoming increasingly popular in many different markets. LEDsprovide a number of advantages over traditional light sources such asincandescent and fluorescent light bulbs, including low powerconsumption, long lifetime, lack of hazardous materials, and additionalspecific advantages for different applications. When used for generalillumination, LEDs provide the opportunity to adjust the color (e.g.,from white, to blue, to green, etc.) or the color temperature (e.g.,from “warm white” to “cool white”) to produce different lightingeffects. In addition, LEDs are rapidly replacing the Cold CathodeFluorescent Lamps (CCFL) conventionally used in many displayapplications (such as LCD backlights), due to the smaller form factorand wider color gamut provided by LEDs. Organic LEDs (OLEDs), which usearrays of multi-colored organic LEDs to produce light for each displaypixel, are also becoming popular for many types of display devices.

LED devices may combine different colors of LEDs within the same packageto produce a multi-colored LED device, or lamp. An example of amulti-colored LED device is one in which two or more different colors ofLEDs are combined to produce white or near-white light. There are manydifferent types of white light lamps on the market, some of whichcombine red, green and blue (RGB) LEDs, red, green, blue and yellow(RGBY) LEDs, white and red (WR) LEDs, RGBW LEDs, etc. By combiningdifferent colors of LEDs within the same package, and driving thedifferently colored LEDs with different drive currents, these lamps maybe configured to generate white light or near-white light within a widegamut of color points or color temperatures ranging from “warm white”(e.g., roughly 2600K-3700K), to “neutral white” (e.g., 3700K-5000K) to“cool white” (e.g., 5000K-8300K).

Although LEDs have many advantages over conventional light sources, adisadvantage of LEDs is that their output characteristics tend to varyover temperature, process and time. For example, it is generally knownthat the luminous flux, or the perceived power of light emitted by anLED, is directly proportional to the drive current supplied thereto. Inmany cases, the luminous flux of an LED is controlled byincreasing/decreasing the drive current supplied to the LED tocorrespondingly increase/decrease the luminous flux. However, theluminous flux generated by an LED for a given drive current does notremain constant over temperature and time, and gradually decreases withincreasing temperature and as the LED ages over time. Furthermore, theluminous flux tends to vary from batch to batch, and even from one LEDto another in the same batch, due to process variations.

LED manufacturers try to compensate for process variations by sorting orbinning the LEDs based on factory measured characteristics, such aschromaticity (or color), luminous flux and forward voltage. However,binning alone cannot compensate for changes in LED outputcharacteristics due to aging and temperature fluctuations during use ofthe LED device. In order to maintain a constant (or desired) luminousflux, it is usually necessary to adjust the drive current supplied tothe LED to account for temperature variations and aging effects.

As discussed further below, such adjustment may involve compensationmeasurements of one or more LED elements within a lamp. Interferencefrom a nearby lamp can cause errors in such measurements for a givenlamp, potentially resulting in incorrect compensation for the lamp. Itwould therefore be desirable to develop interference-resistantcompensation methods for LED illumination devices, and illuminationdevices incorporating such methods.

SUMMARY

The following description of various embodiments of an illuminationdevice and a method for controlling an illumination device is not to beconstrued in any way as limiting the subject matter of the appendedclaims.

A method is provided herein for controlling an illumination devicecomprising multiple emitter modules, where each emitter module comprisesmultiple emission light emitting diodes (LED) elements. An “LED element”as used herein refers to either a single LED or a chain of seriallyconnected LEDs supplied with the same drive current. An “emission LEDelement” as used herein is an LED element configured for light emission,as opposed to, for example, an LED configured as a light detector. Anembodiment of the method includes operating one or more of the multipleemission LED elements in each of the multiple emitter modules at arespective substantially continuous drive current sufficient to produceillumination. The method further includes bringing to a levelinsufficient to produce illumination the respective drive current of allexcept one of the emission LED elements within a first emitter module ofthe multiple emitter modules, for the duration of a first measurementinterval within a first series of measurement intervals interspersedwith periods of illumination. In addition, an embodiment of the methodincludes bringing to a level insufficient to produce illumination therespective drive current of all except one of the emission LED elementswithin a second emitter module of the multiple emitter modules, for theduration of a measurement interval within a second series of measurementintervals interspersed with periods of said operating. The first seriesof measurement intervals and second series of measurement intervals areseparated by a respective first offset and second offset from a timingreference. In an embodiment, the timing reference comprises a periodictiming signal. In a further embodiment, the timing reference is derivedfrom an AC mains signal. In another embodiment, the multiple emittermodules consist of one or more sets of three emitter modules, and eachemitter module within a set uses a respective series of measurementintervals having a different offset from the timing reference than thatused by the other emitter modules within the set.

The method may further include, for either of the first or secondemitter modules, applying to the one of the emission LED elements adrive current sufficient to produce illumination during the measurementinterval within the respective first or second series of measurementintervals, and monitoring a respective first or second measurementphotocurrent induced in a respective first or second measurementphotodetector within the emitter module while the drive current isapplied. In a further embodiment, the method includes, for either of thefirst or second emitter modules, bringing the drive current applied tothe one of the emission LED elements to a level insufficient to produceillumination for a portion of the respective measurement interval, suchthat the respective drive currents of all of the emission LED elementswithin the respective emitter module are at a level insufficient toproduce illumination for the portion of the respective measurementinterval. In such an embodiment, the method may further include, foreither of the first or second emitter modules and during the portion ofthe respective measurement interval, monitoring a respective first orsecond background photocurrent induced in the respective first or secondmeasurement photodetector. In addition, the method may further include,for either of the first or second emitter modules, subtracting therespective first or second background photocurrent from the respectivefirst or second measurement photocurrent. In an embodiment, the resultof this subtraction, for either of the first or second emitter modules,is stored as a respective first or second corrected photocurrent. In afurther embodiment, storing a result of the subtraction is in responseto a determination that the result is within an expected range.

In addition to the method embodiments described above, an illuminationdevice is contemplated herein. In one embodiment, the device includesmultiple emitter modules, where each emitter module includes multipleemission LED elements and one or more photodetectors. The device furtherincludes a control circuit operably coupled to the multiple emittermodules. The control circuit is adapted to operate one or more of themultiple emission LED elements within each of the multiple emittermodules at a respective substantially continuous drive current toproduce illumination. In an embodiment, the control circuit is furtheradapted to bring to a level insufficient to produce illumination therespective drive current of all except one of the emission LED elementswithin a first emitter module of the multiple emitter modules, for theduration of a measurement interval within a first series of measurementintervals interspersed with periods of illumination. The control circuitis further adapted in such an embodiment to bring to a levelinsufficient to produce illumination the respective drive currents ofall except one of the emission LED elements within a second emittermodule of the multiple emitter modules, for the duration of ameasurement interval within a second series of measurement intervalsinterspersed with periods of illumination. The first series ofmeasurement intervals and second series of measurement intervals areseparated by a respective first offset and second offset from a timingreference.

In a further embodiment, the illumination device also includes a timingreference generator operatively coupled to the control circuit andadapted to generate the timing reference. In a still further embodiment,the timing reference comprises a periodic timing signal and the timingreference generator comprises a phase-locked loop. In anotherembodiment, the illumination device further includes multiple drivercircuits operably coupled to respective emitter modules of the multipleemitter modules and to the control circuit, and the control circuit isconfigured to adjust a drive current of an LED element within an emittermodule by providing a drive current setting to a respective drivercircuit for the emitter module.

In another embodiment, the control circuit is further adapted to, foreach of the first and second emitter modules, apply to the one of theemission LED elements a drive current sufficient to produce illuminationduring the measurement interval within the respective first or secondseries of measurement intervals, and monitor a respective first orsecond measurement photocurrent induced in a respective first or secondmeasurement photodetector within the emitter module during the time thedrive current sufficient to produce illumination is applied. In afurther embodiment, the control circuit is further adapted to, for eachof the first and second emitter modules, bring the drive current appliedto the one of the emission LED elements to a level insufficient toproduce illumination for a portion of the respective measurementinterval, such that the respective drive currents of all of the emissionLED elements within the respective emitter module are at a levelinsufficient to produce illumination for the portion of the respectivemeasurement interval. The control circuit may be further adapted tomonitor a respective first or second background photocurrent induced inthe respective first or second measurement photodetector during theportion of the respective measurement interval. In a further embodiment,the control circuit is further adapted to, for each of the first andsecond emitter modules, subtract the respective first or secondbackground photocurrent from the respective first or second measurementphotocurrent.

In a further embodiment, the illumination device also includes aplurality of storage locations accessible by the control circuit, andthe control circuit is further adapted to store a result of subtractingthe first or second background photocurrent from the first or secondmeasurement photocurrent in one or more of the storage locations as afirst or second corrected photocurrent. In a still further embodiment,the control circuit is further adapted to determine whether the resultof the subtraction is within an expected range and store the result inresponse to a determination that the result is within an expected range.In another embodiment, the control circuit includes a respective modulecontrol circuit for each emitter module within the illumination device.In a further embodiment, the control circuit also includes a devicecontrol circuit adapted to provide to each of the module controlcircuits a respective offset from the timing reference for therespective series of measurement intervals used by the respectiveemitter module. In still another embodiment, the multiple emittermodules consist of one or more sets of three emitter modules, and thecontrol circuit is further adapted to use, for each emitter modulewithin a set, a respective measurement interval having a differentoffset from the timing reference than that of the other emitter moduleswithin the set.

BRIEF DESCRIPTION OF THE DRAWINGS

Other objects and advantages of the invention will become apparent uponreading the following detailed description and upon reference to theaccompanying drawings.

FIG. 1 is a graph of the 1931 CIE chromaticity diagram illustrating thegamut of human color perception and the gamut achievable by anillumination device comprising a plurality of multiple color LEDs (e.g.,red, green and blue);

FIG. 2 is a graph illustrating the non-linear relationship betweenrelative luminous flux and junction temperature for white, blue andgreen LEDs;

FIG. 3 is a graph illustrating the substantially more non-linearrelationship between relative luminous flux and junction temperature forred, red-orange and yellow (amber) LEDs;

FIG. 4 is a graph illustrating the non-linear relationship betweenrelative luminous flux and drive current for red and red-orange LEDs;

FIG. 5 is a graph illustrating the substantially more non-linearrelationship between relative luminous flux and drive current for white,blue and green LEDs;

FIG. 6 is an exemplary timing diagram for an illumination devicecomprising four emission LEDs, illustrating intervals during whichemitter forward voltage measurements are obtained from each emissionLED, one LED at a time;

FIG. 7 is a graphical representation depicting how one or moreinterpolation technique(s) may be used in a compensation method todetermine the drive current needed to produce a desired luminous fluxfor a given LED using previously-obtained calibration values storedwithin the illumination device;

FIG. 8 is an exemplary timing diagram for an illumination devicecomprising four emission LEDs and one or more photodetectors,illustrating intervals during which measurements are taken ofphotocurrent, detector forward voltage and emitter forward voltage;

FIG. 9 is a graphical representation depicting how one or moreinterpolation technique(s) may be used in a compensation method todetermine the expected photocurrent value for a given LED using thepresent forward voltage, the present drive current andpreviously-obtained calibration values stored within the illuminationdevice;

FIG. 10 is an exemplary timing diagram illustrating an embodiment forwhich the measurement intervals of FIG. 6 or FIG. 8 are withincompensation periods occurring relatively infrequently, and for whichillumination drive currents are increased during a compensation periodto avoid flicker;

FIG. 11A is a graph illustrating subtraction of ambient light detectedwhen the measured LED element is turned off;

FIG. 11B is a graph illustrating error that can result from ambientsubtraction when a nearby lamp is performing compensation measurements;

FIG. 12 is an exemplary timing diagram illustrating overlap ofcompensation measurements by neighboring lamps;

FIG. 13A is an exemplary timing diagram illustrating a series ofdetection intervals followed by a series of measurement intervals;

FIG. 13B is a timing diagram illustrating a series of detectionintervals interspersed with intervals for taking non-sensitivemeasurements, followed by a series of intervals for taking sensitivemeasurements;

FIG. 14 is an exemplary timing diagram illustrating overlapping butnon-interfering measurement sequences by neighboring lamps;

FIG. 15 is an exemplary timing diagram illustrating a timing referencesynchronized to the AC mains, and first and second sets of measurementintervals separated from the timing reference by first and second offsettimes;

FIG. 16A is a flow chart illustrating an exemplary method disclosed forcontrolling a lamp to perform compensation measurements;

FIG. 16B is a flow chart illustrating an exemplary method forcontrolling a lamp to initiate compensation measurements;

FIG. 16C is a flow chart illustrating another exemplary method forcontrolling a lamp to initiate compensation measurements;

FIG. 17 is a chart illustrating exemplary configuration information thatmay be stored within an illumination device and used in embodiments ofmethods described herein;

FIG. 18A is a photograph of an exemplary multi-lamp illumination device;

FIG. 18B is a computer generated image showing a top view of anexemplary emitter module, or lamp, that may be included within theexemplary illumination device of FIG. 18A;

FIG. 19A is a photograph of an exemplary illumination device;

FIG. 19B is a computer generated image showing a top view of anexemplary emitter module, or lamp, that may be included within theexemplary illumination device of FIG. 19A;

FIG. 20 is an exemplary block diagram of circuit components that may beincluded within an embodiment of an illumination device disclosedherein;

FIG. 21 is an exemplary block diagram of an embodiment of an LED driverand receiver circuit that may be included within the illumination deviceof FIG. 20;

FIG. 22 is an exemplary block diagram of circuit components that may beincluded within an embodiment of a multi-lamp illumination devicedisclosed herein; and

FIG. 23 is an exemplary block diagram of an embodiment of interface andemitter circuitry that may be included within the illumination device ofFIG. 22.

While the invention is susceptible to various modifications andalternative forms, specific embodiments thereof are shown by way ofexample in the drawings and will herein be described in detail. Itshould be understood, however, that the drawings and detaileddescription thereto are not intended to limit the invention to theparticular form disclosed, but on the contrary, the intention is tocover all modifications, equivalents and alternatives falling within thespirit and scope of the present invention as defined by the appendedclaims.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

An LED generally comprises a chip of semiconducting material doped withimpurities to create a p-n junction. As in other diodes, current flowseasily from the p-side, or anode, to the n-side, or cathode, but not inthe reverse direction. Charge-carriers—electrons and holes—flow into thejunction from electrodes with different voltages. When an electron meetsa hole, it falls into a lower energy level, and releases energy in theform of a photon (i.e., light). The wavelength of the light emitted bythe LED, and thus its color, depends on the band gap energy of thematerials forming the p-n junction of the LED.

Red and yellow LEDs are commonly composed of materials (e.g., AlInGaP)having a relatively low band gap energy, and thus produce longerwavelengths of light. For example, most red and yellow LEDs have a peakwavelength in the range of approximately 610-650 nm and approximately580-600 nm, respectively. On the other hand, green and blue LEDs arecommonly composed of materials (e.g., GaN or InGaN) having a larger bandgap energy, and thus, produce shorter wavelengths of light. For example,most green and blue LEDs have a peak wavelength in the range ofapproximately 515-550 nm and approximately 450-490 nm, respectively.

In some cases, a “white” LED may be formed by covering or coating, e.g.,a violet or blue LED having a peak emission wavelength of about 400-490nm with a phosphor (e.g., YAG), which down-converts the photons emittedby the blue LED to a lower energy level, or a longer peak emissionwavelength, such as about 525 nm to about 600 nm. In some cases, such anLED may be configured to produce substantially white light having acorrelated color temperature (CCT) of about 3000K. However, a skilledartisan would understand how different colors of LEDs and/or differentphosphors may be used to produce a “white” LED with a potentiallydifferent CCT.

When two or more differently colored LEDs are combined within a singlepackage, the spectral content of the individual LEDs is combined toproduce blended light. In some cases, differently colored LEDs may becombined to produce white or near-white light within a wide gamut ofcolor points or CCTs ranging from “warm white” (e.g., roughly2600K-3000K), to “neutral white” (e.g., 3000K-4000K) to “cool white”(e.g., 4000K-8300K). Examples of white light illumination devicesinclude, but are not limited to, those that combine red, green and blue(RGB) LEDs, red, green, blue and yellow (RGBY) LEDs, white and red (WR)LEDs, and RGBW LEDs.

The illumination devices disclosed herein may in certain embodimentsinclude one or more emitter modules, which may also be called lamps. Anemitter module has a plurality of LED elements and one or morephotodetectors combined into a package. As noted above, an LED elementmay be either a single LED or a chain of serially connected LEDssupplied with the same drive current. An LED element configured for itsjunction(s) to have sufficient forward bias for light emission may bereferred to herein as an “emission LED element.” An LED may also beconfigured as a photodetector, typically by applying zero bias orreverse bias to the LED junction and collecting photocurrent induced byincident light. In an embodiment, multiple LEDs configured asphotodetectors may be connected in parallel so that their photocurrentscan be combined.

Although not limited to such, the present invention is particularly wellsuited to multi-colored illumination devices in which two or moredifferent colors of LEDs are combined to produce blended white ornear-white light, since the output characteristics of differentlycolored LEDs vary differently over drive current, temperature and time.The present invention is also particularly well suited to illuminationdevices (i.e., tunable illumination devices) that enable the targetdimming level and/or the target chromaticity setting to be changed byadjusting the drive currents supplied to one or more of the LEDs, sincechanges in drive current inherently affect the lumen output, color andtemperature of the illumination device. These tunable illuminationdevices should all produce the same color and color rendering index(CRI) when set to a particular dimming level and chromaticity setting(or color set point) on a standardized chromaticity diagram.

A chromaticity diagram maps the gamut of colors the human eye canperceive in terms of chromaticity coordinates and spectral wavelengths.An example of a chromaticity diagram is shown in FIG. 1. The spectralwavelengths of all saturated colors are distributed around the edge ofan outlined space (called the “gamut” of human vision), whichencompasses all of the hues perceived by the human eye. The curved edgeof the gamut is called the spectral locus and corresponds tomonochromatic light, with each point representing a pure hue of a singlewavelength. The straight edge on the lower part of the gamut is calledthe line of purples. These colors, although they are on the border ofthe gamut, have no counterpart in monochromatic light. Less saturatedcolors appear in the interior of the figure, with white and near-whitecolors near the center.

In the 1931 Commission Internationale de l'Êclairage (CIE) ChromaticityDiagram of FIG. 1, colors within the gamut of human vision are mapped interms of chromaticity coordinates (x, y). The diagram of FIG. 1 is onlyone illustrative example of how perceived colors may be representedusing a two-dimensional space, and other “color spaces,” withcorresponding chromaticity values, may also be used. Some exemplarycolor spaces include the CIE 1931 XYZ color space, the CIE 1931 RGBcolor space, the CIE 1976 LUV color space, and various other RGB colorspaces (e.g., sRGB, Adobe RGB, etc.). Wavelength in nanometers (nm) ofthe corresponding monochromatic light is indicated along the curved edgeof the gamut in FIG. 1. The dominant wavelength, as perceived by theeye, of a point within the gamut may be found using a line including thepoint and a reference point for the illumination source, such as point Cof FIG. 1 corresponding to the CIE-C reference. The dominant wavelengthunder the reference illumination is read at the intersection of the linewith the curved edge of the gamut. For example, a red (R) LED with adominant wavelength of about 640 nm may have a chromaticity coordinateof (0.68, 0.28), a green (G) LED with a dominant wavelength of about 525nm may have a chromaticity coordinate of (0.17, 0.72), and a blue (B)LED with a dominant wavelength of 465 nm may have a chromaticitycoordinate of (0.16, 0.11). This dominant wavelength perceived by theeye does not necessarily correspond to the peak wavelength, orwavelength of highest intensity, emitted from an LED.

The color of an incandescent black body as a function of temperature inKelvin is also plotted on the diagram of FIG. 1, in a curve known as theblackbody locus. The chromaticity coordinates (i.e., color points) thatlie along the blackbody locus obey Planck's equation,E(λ)=Aλ⁻⁵/(e^((B/T))−1). Color points that lie on or near the blackbodylocus provide a range of white or near-white light with colortemperatures ranging between approximately 2500K and 10,000K. Thesecolor points are typically achieved by mixing light from two or moredifferently colored LEDs. For example, light emitted from the RGB LEDsplotted in FIG. 1 may be mixed to produce a substantially white lightwith a color temperature in the range of about 2500K to about 5000K.

Although an illumination device is typically configured to produce arange of white or near-white color temperatures arranged along theblackbody curve (e.g., about 2500K to 5000K), some illumination devicesmay be configured to produce any color within the color gamut, such astriangular color gamut 18 of FIG. 1, formed by the individual LEDs(e.g., RGB). The chromaticity coordinates of the combined light, e.g.,(0.437, 0.404) for 3000K white light, define the target chromaticity orcolor set point at which the device is intended to operate. In somedevices, the target chromaticity or color set point may be changed byaltering the ratio of drive currents supplied to the individual LEDs.

In general, the target chromaticity of the illumination device may bechanged by adjusting the drive current levels (in current dimming) orduty cycle (in PWM dimming) supplied to one or more of the emissionLEDs. For example, an illumination device comprising RGB LEDs may beconfigured to produce “warmer” white light by increasing the drivecurrent supplied to the red LEDs and decreasing the drive currentssupplied to the blue and/or green LEDs. Since adjusting the drivecurrents also affects the lumen output and temperature of theillumination device, the target chromaticity must be carefullycalibrated and controlled to ensure that the actual chromaticity equalsthe target value.

FIGS. 2-3 illustrate how the relative luminous flux of an individual LEDchanges over junction temperature for different colors of LEDs. As shownin FIGS. 2-3, the luminous flux output from all LEDs generally decreaseswith increasing temperature. For some colors (e.g., white, blue andgreen), the relationship between luminous flux and junction temperatureis relatively linear (see FIG. 2), while for other colors (e.g., red,orange and especially yellow) the relationship is significantlynon-linear (see, FIG. 3). The chromaticity of an LED also changes withtemperature, due to shifts in the dominant wavelength (for both phosphorconverted and non-phosphor converted LEDs) and changes in the phosphorefficiency (for phosphor converted LEDs). In general, the peak emissionwavelength of green LEDs tends to decrease with increasing temperature,while the peak emission wavelength of red and blue LEDs tends toincrease with increasing temperature. While the change in chromaticityis relatively linear with temperature for most colors, red and yellowLEDs tend to exhibit a more significant non-linear change.

FIGS. 4 and 5 illustrate the relationship between luminous flux anddrive current for different colors of LEDs (e.g., red, red-orange,white, blue and green LEDs). In general, the luminous flux increaseswith larger drive currents, and decreases with smaller drive currents.However, the change in luminous flux with drive current is non-linearfor all colors of LEDs, and this non-linear relationship issubstantially more pronounced for certain colors of LEDs (e.g., blue andgreen LEDs) than others. The chromaticity of the illumination alsochanges when drive currents are increased to combat temperature and/oraging effects, since larger drive currents inherently result in higherLED junction temperatures (see, FIGS. 2-3). While the change inchromaticity with drive current/temperature is relatively linear for allcolors of LEDs, the rate of change is different for different LED colorsand even from part to part.

U.S. application Ser. Nos. 13/970,990 and 14/314,530, co-pending withthe present application and commonly owned and/or subject to assignmentwith the present application, describe methods of compensation forvariation in quantities including temperature and drive current, andillumination devices employing such methods. Approaches described inthese applications to compensating for variations in luminous flux fromLEDs, such as the effects illustrated by FIGS. 2-5, in some embodimentsinclude the use of calibration tables created for the LEDs within anillumination device. Such calibration tables store results ofcalibration measurements previously made using the LEDs. In anembodiment, a calibration table stores values of photocurrent induced ona photodetector within the illumination device when a drive current isapplied to each LED within the device separately. Such a calibrationtable may in some embodiments store photocurrent values obtained whenapplying multiple different drive current levels to an LED. In someembodiments in which photocurrent values are obtained when applyingdifferent drive current levels, forward voltage measurements areobtained for each LED after each drive current is applied. Such forwardvoltage measurements can be used as an indication of junctiontemperature in the LED. The calibration table may in further embodimentsstore photocurrent values obtained at different values of ambienttemperature. Other types of data and variations of the above-describeddata may also be included in a calibration table, as described in moredetail in co-pending application Ser. Nos. 13/970,990 and 14/314,530. Ingeneral, the data stored in a calibration table is in some embodimentsused for comparison to measurements made during operation of theillumination device. Such comparison can be used to indicate whetherproperties of one or more of the LEDs within the device have changed,and whether the corresponding drive current of the LED should beadjusted.

Exemplary compensation approaches for an illumination device includingmultiple emission LED elements and at least one photodetector areillustrated by FIGS. 6-8. FIG. 6 is an exemplary timing diagramillustrating substantially continuous operation of one or more of theLED elements to produce illumination. As used herein, the term“substantially continuously” means that an operative drive current(denoted generically as I1 in FIG. 6) is supplied to the emission LEDelements almost continuously, with the exception of intervals in whichall of the emission LED elements are momentarily turned “off” for shortdurations of time 610. As used herein, “off” in connection with an LEDelement refers to the LED element having a drive current reduced to anon-operative level, such that the LED element does not produceillumination that is generally detectable by the detectors used in theillumination device or in nearby devices. In an embodiment, drivecurrent I1 represents a combination of different drive currents appliedas appropriate to respective different LED elements within theillumination device, to produce the desired illumination. In theexemplary embodiment of FIG. 6, the intervals are utilized for obtainingforward voltage measurements from each of four emission LED elements(Vfe), one LED element at a time, by supplying a relatively small drivecurrent to each LED and measuring the forward voltage developedthere-across. The intervals may also be used for other types ofmeasurements, as shown in FIGS. 8-9 and discussed in more detail below.In certain embodiments discussed further below, all LED elements withinthe illumination device remain off throughout some of the intervals toallow detection to determine whether measurements are being conducted bya different illumination device.

In the embodiment of FIG. 6, the illumination device includes at leastfour emission LED elements. In an embodiment, the device includesexactly four emission LED elements, and the forward voltage across eachelement is measured, one at a time during successive respectivemeasurement intervals. Unless specified otherwise, a measurementperformed “during” an interval as used herein is performed within theinterval, but not necessarily for the entirety of the interval. In suchan embodiment the four emission LED elements may be of different colorsto form a multi-color lamp. In some embodiments the multicolor lamp maybe configured to produce white light, as described above. Duringillumination periods 620, one or more of the LED elements are drivenwith respective DC drive currents to produce illumination. In anembodiment, all of the LED elements in the lamp are driven duringillumination periods 620. In other embodiments, depending on the color,intensity, and/or pattern of light desired, fewer than all of the LEDelements may be driven during the illumination periods. With theexception of the LED under test, all emission LED elements within thedevice are turned off throughout intervals 610, however, with theirrespective drive currents removed or at least reduced to non-operativelevels (denoted as I0 in FIG. 6). In an embodiment, intervals 610 arepart of a periodic series having a specific offset (which may be zero)from a periodic timing reference.

The plot in FIG. 7 of luminous flux vs. LED drive current illustrates anexemplary technique of using calibration values to determine the drivecurrent (Ix) needed to achieve a desired luminous flux (Lx) from anemission LED element at its present operating temperature (reflected inthe present value of Vfe, Vfe_present, for the LED element measuredduring one of intervals 610 of FIG. 6). Data points 710, denoted byfilled circles, represent luminous flux values from a calibration table,obtained during calibration of the LED element using three differentdrive currents (10%, 30% and 100% of the maximum drive current, in theembodiment of FIG. 7) and two different ambient temperatures T0 and T1.Each of data points 710 may be associated with a respective forwardvoltage value Vfe in the calibration table, obtained just before or justafter the respective luminous flux measurement at the respective drivecurrent and ambient temperature value. Comparison of these forwardvoltages in the calibration table for a given LED element to a forwardvoltage measured during operation can allow the present temperatureT_present to be estimated. In an embodiment, interpolation between thecalibration values 710 is used to predict luminous flux values 720,denoted by unfilled triangles, corresponding to the calibration drivecurrents at the current operating temperature (T_present). In a furtherembodiment, an interpolation or curve-fitting using predicted values 720is used to generate a relationship, plotted as curve 730, for luminousflux vs. drive current at the present operating temperature. The drivecurrent Ix needed to produce the desired luminous flux Lx can then beobtained from the generated relationship. As described further in theabove-referenced co-pending applications, the specific interpolationtechniques used may depend on the characteristics of the LED elementbeing compensated, along with considerations such as memory andprocessing capability. The approach illustrated in FIGS. 6 and 7 isemployed in embodiments of methods for maintaining a target luminousflux from an LED element in spite of changes in the LED element'stemperature.

Another example of a compensation method is illustrated by FIGS. 8 and9. The timing diagram of FIG. 8 is similar to that of FIG. 6, withoperative drive current I1 supplied to one or more of the emission LEDelements within an illumination device almost continuously, with theexception of intervals during which all of the emission LED elements,except for the emission LED under test, are momentarily turned off forshort durations of time 810. In the embodiment of FIG. 8, the first fourof intervals 810 are used for measuring a photocurrent (Iph) induced ona photodetector within the illumination device, in response toillumination that is produced by each emission LED element, one LEDelement at a time. During each photocurrent measurement, the emissionLED under test is driven with an operative drive current level. In anembodiment, such photocurrent measurements allow detection of changes inthe luminous flux produced by an LED element at a given drive current,as may occur in LEDs over time.

The plot in FIG. 9 of photocurrent induced on a detector as a functionof LED drive current illustrates an exemplary technique of usingcalibration values to determine the expected photocurrent (Iph_exp)induced by a particular drive current (Ix) applied to an emission LEDelement at the present detector temperature (reflected in the presentvalue of the forward voltage measured across the detector, Vfd_present,during one of intervals 810 of FIG. 8). Data points 910, denoted byfilled circles, represent photocurrent values from a calibration table,obtained during calibration of an LED element using three differentdrive currents (10%, 30% and 100% of the maximum drive current, in theembodiment of FIG. 9) and two different ambient temperatures(corresponding to Vfd0 and Vfd1 measured at ambient temperatures T0 andT1). In an embodiment, interpolation between the calibration values isused to predict expected photocurrent values 920, denoted by unfilledtriangles, corresponding to the calibration drive currents at thecurrent detector temperature (Vfd_present). In a further embodiment, aninterpolation or curve-fitting using predicted values 920 is used togenerate a relationship, plotted as curve 930, for expected photocurrentvs. drive current at the present detector temperature. The expectedphotocurrent induced on the detector by an LED operated at the presentvalue of drive current (for example, a drive current obtained using themethod illustrated in FIGS. 6 and 7) can then be obtained from thegenerated relationship. This expected value can then be compared to thecorresponding presently measured photocurrent obtained during one ofintervals 810 shown in FIG. 8. In an embodiment of a compensationmethod, a difference between the measured and expected values indicatesa change in the light intensity generated by the LED element over time.Such an “aging” effect may be compensated for by adjusting the drivecurrent applied to the LED element, as described in co-pendingapplication Ser. No. 14/314,530.

FIGS. 6-9 illustrate two examples of compensation methods. As discussedfurther in the above-referenced co-pending applications, othercompensation methods may be used instead of or in combination with thesemethods. For example, variations in additional quantities, such as x andy chromaticity values, can be compensated for. In some embodiments,adjustment to compensate for one quantity may cause a variation inanother, such that compensation methods are iterated until stabledesired settings are achieved. Other embodiments of compensation methodsmay also include taking additional or different measurements than thoseindicated in FIGS. 6 and 8. For example, photocurrent measurements mayinclude measurements using each of multiple photodetectors, where eachphotodetector is configured for sensitivity to a different spectralrange.

As shown by the examples above and described further in the co-pendingapplications referenced herein, it can be advantageous to takemeasurements during brief interruptions in illumination by an LEDillumination device. When used in conjunction with calibration data,such measurements allow monitoring and correction of variations fromdesired settings. In one embodiment, a series of intervals such asintervals 610 of FIG. 6 may extend for the entire time that anillumination device is operating. In such an embodiment, a sequence ofcompensation measurements may be repeated continuously, one measurementper interval, while the illumination device is operating.

In an alternative embodiment, compensation using intervals such asintervals 610 of FIG. 6 is performed only at certain times duringoperation of an illumination device. For example, compensation may beperformed when a significant change in ambient temperature has beendetected, or when there has been a change in settings for theillumination device. Timing diagrams illustrating performance ofcompensation at selected times are shown in FIG. 10. The upper diagramof FIG. 10 illustrates periods 1010 of continuous illumination producedby application of an operative drive current designated I1 to one ormore LED elements. In an embodiment, drive current I1 represents acombination of different drive currents applied to respective differentLED elements within the illumination device, to produce the desiredillumination. In the embodiment of FIG. 10, illumination periods 1010are occasionally interrupted by compensation periods 1020, during whichmeasurements are taken as part of a compensation method. In anembodiment, initiation of a compensation period 1020 is in response to adetermination that there has been a change in some quantity such asambient temperature or illumination settings for the device. In such anembodiment, compensation periods may be repeated until a changingquantity has stabilized. In an alternative embodiment, compensationperiods 1020 may be initiated at previously specified times or for afixed number of times, including one time.

The lower diagram of FIG. 10 is an expanded timing diagram of anexemplary compensation period 1020. Intervals 1022 are similar tointervals 610 of FIG. 6 or intervals 810 of FIG. 8. Within intervals1022, all emission LED elements are turned off except for a single LEDelement that may be turned on as part of a particular measurement.Between intervals 1022, one or more of the LED elements within the lampare supplied with an operative drive current during illumination periods1024. In the embodiment of FIG. 10, the drive current applied duringillumination periods 1024 is “boosted” to an increased level designatedgenerically as 12. In an embodiment, drive current level I2 represents acombination of different drive currents applied to respective differentLED elements, each at a higher level than is applied to the LED elementin connection with drive current level I1 during illumination periods1010. As discussed in more detail in co-pending application Ser. No.13/970,990, use of a boosted drive current during compensation periodsmay counteract a “flicker” effect that can result from the interruptionsin illumination occurring during a compensation period such as period1020.

As discussed above in connection with FIGS. 8-9, in some embodimentscompensation methods for an LED illumination device such as an emittermodule rely upon measurements of photocurrent induced in a photodetectorwhen a drive current is applied to an LED element. In such anembodiment, it is critical that the photocurrent induced reflect the LEDelement being measured rather than interference from other lightsources. In some embodiments of methods disclosed herein, subtraction ofambient-induced photocurrent is employed to mitigate the effects ofinterference. An embodiment for which interference-related illuminationcan be effectively subtracted is illustrated in FIG. 11A.

The upper diagram of FIG. 11A plots luminous flux vs. time during aninterval 1102 similar to, for example, interval 1022 of FIG. 10. In theembodiment of FIG. 11A, a first portion 1104 of the interval is ameasurement portion of the interval during which a particular emissionLED element may be turned on (while all other emission LED elements inthe illumination device are turned off). Second portion 1106 in thisembodiment is a portion of the interval used for ambient detection,during which all emission LED elements within the illumination deviceare turned off. Although portions 1104 and 1106 each have a duration ofapproximately one-half of interval 1102, the portions could havedifferent relative durations in other embodiments. Waveform 1110,denoted with a solid line, represents the luminous flux resulting fromturning on an LED element during interval portion 1104 for ameasurement, then turning the LED element off during interval portion1106. Waveform 1112, denoted with a dashed line, represents the luminousflux resulting from ambient light that is constant in intensity for atleast the duration of interval 1102.

The lower diagram of FIG. 11A plots photocurrent induced in aphotodetector in response to the luminous flux plotted in the upperdiagram. For purposes of illustration, it is assumed that thephotodetector has equal sensitivity to the LED illumination representedby waveform 1110 and the ambient illumination represented by waveform1112. Waveform 1114, denoted with a solid line, represents the totalphotocurrent induced by the LED and ambient illumination, or the sum ofthe photocurrent induced by each type of illumination. Waveform 1116,denoted by a dashed line, represents the difference between the totalphotocurrent at any time and an ambient current value I_(A), where I_(A)is the total current measured at a point during portion 1106 of interval1102. For example, I_(A) corresponds to the total photocurrent at timeT_(A). In other embodiments, I_(A) can be obtained by averaging multiplemeasurements taken during interval portion 1106, or by using othersignal processing techniques known to one of ordinary skill in the artin view of this disclosure. Similarly, total photocurrent I_(T) isobtained by one or more measurements of photocurrent in the detectorduring interval portion 1104, accompanied by averaging and/or othersignal processing as understood by one of ordinary skill in the art inview of this disclosure. Subtraction of ambient photocurrent I_(A) fromtotal photocurrent I_(T) results in corrected photocurrent I_(C)attributable to the LED illumination corresponding to waveform 1110.

In an embodiment, the detector used to measure induced ambientphotocurrent I_(A) is the same detector used to measure totalphotocurrent I_(T) during interval portion 1104 when the target LEDelement is driven at an operative current level. In this way, theambient photocurrent induced during measurement of the tested LEDelement may be most accurately accounted for by the ambient photocurrentdetected during interval portion 1106 when the tested LED element isoff. In some embodiments, a separate detector may be used for ambientlight detection, alternatively or in addition to a detector used forambient detection during photocurrent measurements. A separate detectorfor ambient light measurement may be particularly useful, for example,in embodiments for which target settings of the illumination device areadjusted depending on ambient light conditions.

The importance of the ambient subtraction of FIG. 11A can be appreciatedby reference back to the method illustrated by FIGS. 8-9. As describedabove, FIG. 9 illustrates determination of an expected photocurrentvalue by interpolation from stored calibration values. The expectedvalue is compared to the photocurrent measured for the corresponding LEDelement—for example, Iph1 of FIG. 8. If the measured photocurrentincludes photocurrent induced by illumination other than that from theLED element, such as total current I_(T) of FIG. 11A, comparison to theexpected photocurrent determined as shown in FIG. 9 will provide aninaccurate indication of how illumination from the LED element haschanged. The resulting scaling and adjustment of drive current to theLED element may therefore move the LED element away from its targetsettings rather than helping to maintain them. Comparison of theexpected photocurrent to corrected photocurrent I_(C) in the embodimentof FIG. 11A, however, should provide an accurate indication of how theillumination from the LED element may have changed.

A situation in which the subtraction technique illustrated in FIG. 11Ais not effective in mitigating interference is illustrated by FIG. 11B.The upper diagram of FIG. 11B is a plot of luminous flux during the sameinterval 1102 having first and second portions 1104 and 1106,respectively, as that shown in the upper diagram of FIG. 11A. The upperdiagram also includes waveform 1110 as also shown in FIG. 11A,representing luminous flux from an LED element turned on during intervalportion 1104. Instead of the constant ambient illumination 1112 shown inFIG. 11A, however, the upper diagram of FIG. 11B includes waveform 1120representing an additional illumination source that is on duringinterval portion 1104 and off during interval portion 1106. In anembodiment, waveform 1120 represents illumination from an additional LEDelement within a separate illumination device or emitter module thanthat of the LED element represented by waveform 1110.

The lower diagram of FIG. 11B plots photocurrent induced in aphotodetector in response to the luminous flux plotted in the upperdiagram, assuming equal sensitivity of the photodetector to the LEDillumination represented by waveforms 1110 and 1120. Like waveform 1114of FIG. 11A, waveform 1122 in FIG. 11B represents the total photocurrentinduced by the illumination sources corresponding to waveforms 110 and1120. In the embodiment of FIG. 11B, the difference between the totalphotocurrent and current I_(A) measured at a point during portion 1106of interval 1102 is also represented by waveform 1122, because I_(A) iszero in FIG. 11B. Using I_(A), I_(T) and I_(C) defined in the samemanner as for FIG. 11A, I_(C) is equal to I_(T) in the embodiment ofFIG. 11B because I_(A) is zero. Therefore, I_(C) in FIG. 11B does notrepresent the photocurrent induced solely by illumination from the LEDelement corresponding to waveform 1110. Use of the photocurrent fromFIG. 11B in a compensation method such as that illustrated in FIGS. 8and 9 would lead to serious errors since a photocurrent notcorresponding to a given LED element would be used for determining theadjustment to the drive current of that LED element.

In the example of FIG. 11B, an extreme case is illustrated of aninterfering light source that is turned on and off at exactly the sametimes as the LED element being compensated. It is noted that anyinterference source not having constant intensity over interval 1102 canproduce an error in measured photocurrent, even if the interferencesource does not turn on and off at exactly the same times as the targetLED element. If the “ambient” photocurrent measured during intervalportion 1106 is not equal to the interference-generated portion of thephotocurrent measured during interval portion 1104, ambient subtractionwill not be effective in extracting the photocurrent corresponding tothe LED element being compensated. An embodiment including anon-constant interference source as shown in FIG. 11B may of courseinclude constant ambient illumination as well, in the manner shown inFIG. 11A. In such an embodiment, the photocurrent associated with theconstant illumination could be subtracted out, while the non-constantinterfering illumination would lead to compensation errors.

“Non-constant illumination” as used herein refers to illumination havinga substantial variation with time during a measurement interval, orduring a portion of a measurement interval in which detection ofbackground or ambient illumination is being performed. In an embodiment,a substantial variation is a variation that would result in asignificant error for a photocurrent measurement conducted during thesame interval. The size of the variation that would result in asignificant error depends on the relative magnitudes of photocurrentsinduced by a measured LED element and by the external illumination inthe photodetector used for the photocurrent measurement.

A further illustration of how the kind of interference shown in FIG. 11Bcan arise is given by FIG. 12. Two timing diagrams are shown in FIG. 12.The upper diagram, designated Lamp A, is associated with a first emittermodule including multiple LED elements and a photodetector. The lowerdiagram, designated Lamp B, corresponds to a second emitter module. Thetwo lamps may in some embodiments be part of a single largerillumination device. In other embodiments, the two lamps may be inseparate illumination devices that are installed in proximity to oneanother, or even facing one another. Each timing diagram corresponds toa portion of a compensation period such as period 1020 of FIG. 10, inwhich periods of illumination 1202 are interrupted by intervalsincluding intervals 1210, 1220, 1230 and 1240, during which the emissionLED elements within the lamp are turned off and a measurement associatedwith a particular LED element and/or detector may be taken. In someembodiments, drive currents applied to LED elements during theillumination periods may be “boosted” as shown in FIG. 10, to a higherlevel as compared to the level during longer illumination periods notinterrupted by measurements, such as periods 1010 of FIG. 10.

During interval 1210 of FIG. 12, a forward voltage measurement (denotedas V_(f1A)) is taken of an emission LED element 1 within Lamp A. Nomeasurements are taken for Lamp B during interval 1210; instead, drivecurrents are applied to one or more of the emission LED elements of LampB to produce the desired illumination. In other words, interval 1210 isan interval for Lamp A but not for Lamp B. Whether illumination fromLamp B interferes with the forward voltage measurement taken for Lamp Adepends on the relative magnitudes of the bias-induced current in theLED element being measured and the photocurrent induced in the LEDelement by the external illumination. The magnitude of the photocurrentinduced may depend on multiple factors, such as the relative locationsof Lamp B and Lamp A, the relative wavelengths of the driven LED elementin Lamp B and Lamp A, and the carrier recombination lifetimes undermeasurement conditions for the measured LED element in Lamp A. In anembodiment, the induced photocurrent from external radiation is on theorder of a microampere or less, while the forward bias induced currentin the measured element is on the order of a milliampere. In such anembodiment, illumination by Lamp B in interval 1210 of FIG. 12 would nothave a significant effect on the forward voltage measurement taken byLamp A. The forward voltage measurement in such an embodiment may beconsidered to not be sensitive to illumination from the otherillumination device.

In an alternative embodiment in which Lamp A were taking a photocurrentmeasurement during interval 1210 rather than a forward voltagemeasurement, the magnitude of the externally-induced photocurrent may besignificant by comparison to the measured current. However, the constantillumination provided by the illumination from Lamp B during interval1210 could be successfully subtracted out if a photocurrent measurementwere taken by Lamp A during that interval. This subtraction wouldcorrespond to the situation illustrated in FIG. 11A above.

During each of intervals 1220 and 1240, one of the lamps is performing aphotocurrent measurement on an LED element, while the other lamp isperforming a forward voltage measurement. During interval 1240, forexample, a forward voltage measurement V_(f2A) of emission LED element 2of Lamp A is performed, while a photocurrent measurement I_(ph2B)measures the photocurrent induced in a detector of Lamp B by operationof emission LED element 2 of Lamp B. In an embodiment, forward voltagemeasurements of emission LED elements are taken using non-operativelevels of drive current, meaning drive current levels insufficient toproduce significant illumination from the LED. In such an embodiment,the forward voltage measurement taken using one lamp would not beexpected to interfere with the photocurrent measurement taken using theother lamp. Whether there is interference in the oppositedirection—i.e., whether the photocurrent measurement of Lamp Binterferes with the forward voltage measurement of Lamp A—depends uponthe relative magnitudes of the forward bias induced current in themeasured LED element of Lamp A and the photocurrent induced in that LEDelement by the illumination from Lamp B. This can depend on variousfactors, as discussed above in the discussion of interval 1210.

During interval 1230, however, a photocurrent measurement is taken inboth Lamp A and Lamp B. Because illumination is produced by both ofthese measurements, errors will be introduced into each measurement, andany resulting drive current adjustments, to the extent that illuminationproduced by one lamp is detectable by the other lamp. Interference fromthese two photocurrent measurements cannot be mitigated using ambientsubtraction techniques. An attempt to subtract interference-relatedphotocurrent from the photocurrent measured by each lamp would in oneembodiment lead to a situation similar to that shown in FIG. 11B: eachLED element would be turned on during one portion of interval 1230 andoff during the other portion, causing the “corrected” photocurrentvalues to be too large. (Even in an embodiment for which one lamp turnedits LED element on during a first portion of the interval and the otherlamp turned its LED element on during a second portion, the ambientsubtraction would still be incorrect: in this case the “ambient”subtracted would be too large and the resulting “corrected” photocurrenttoo small.) Another way of avoiding interference caused by two lampstaking measurements during the same interval is needed.

In an embodiment of a method described herein for avoiding interference,detection is performed during one or more intervals before aphotocurrent measurement is performed during one of the intervals. In afurther embodiment, the detection during one or more intervals isperformed before any measurement associated with compensation of anillumination device is performed. Photocurrent measurements, or in someembodiments any measurements, are initiated after detection has beenperformed for enough intervals to indicate that interference fromcompensation measurements of another lamp is unlikely. In an embodiment,a photodetector is used to determine whether outside illumination ispresent that is not constant throughout the measurement interval.

In an embodiment, the number of intervals used for detection depends onthe particular sequences of measurements used by the illumination deviceperforming the method and by any potentially interfering devices. Asnoted above in the discussion of FIG. 12, some types of measurement usedfor compensation of LED elements in an illumination device are morelikely to interfere with other illumination devices than other types ofmeasurement. In an embodiment, the specific measurements most likely tocause interference include measurements of photocurrent induced in adetector by an illuminated LED element. In such an embodiment, those arethe measurements most likely to produce a non-constant illumination thatcould interfere with a photocurrent measurement by a differentillumination device. In a typical embodiment, the measurements that aremost likely to result in interference are also the measurements mostlikely to be detected by a different illumination device employingdetection intervals before starting its own photocurrent measurements.The number of intervals used for detection may depend on how many totalmeasurements are expected to be performed in a compensation measurementsequence, as well as how many of those measurements are expected to beof the kind most likely to cause interference.

As an example, consider an emitter module including 4 LED elements andat least one photodetector. The photodetector(s) may be dedicatedphotodetectors or may in some embodiments be emission LEDs configured atcertain times as photodetectors. In an embodiment, such an emittermodule may use a sequence of 12 measurements for compensation. Forexample, 4 of the compensation measurements could be forward voltagemeasurements for each of the 4 LED elements. Another 4 measurementscould be photocurrent measurements for each of the 4 LED elements usingone dedicated photodetector. Another 2 measurements could bephotocurrent measurements for two of the LED elements using anadditional photodetector. The remaining 2 measurements could be forwardvoltages across each of two detectors. In this example, 6 of the 12compensation measurements are photocurrent measurements.

In one embodiment of the above example, it may be expected that anyinterfering illumination devices will also be configured to use asequence of 12 compensation measurements, 6 of which are photocurrentmeasurements. If the particular sequence of measurements that aninterfering device may be configured to use is not known, one approachwould be to detect for 12 measurement intervals before startingcompensation measurements. If no non-constant illumination is detectedduring any of the 12 intervals, it is likely that no nearby illuminationdevice is performing compensation measurements. In another embodiment,if it is expected that 6 of the compensation measurements performed byan interfering device are photocurrent measurements, detection could beperformed for 7 intervals before starting compensation measurements ifno non-constant illumination is detected. If another device wereperforming compensation measurements including six photocurrentmeasurements, one of the 6 photocurrent measurements would be expectedto occur within a sequence of 7 intervals. In still another embodiment,if the 6 photocurrent measurements were expected to be uniformly spacedwithin the 12-measurement sequence (in this case, every othermeasurement of the 12 measurements would be a photocurrent measurement),2 consecutive intervals in which no non-constant illumination isdetected may be sufficient to indicate that no nearby device is likelyto be currently performing compensation measurements.

In a further embodiment of the emitter module example described above,the various photocurrent measurements included in the compensationmeasurement sequence are not equally detectable. Some of thephotocurrent measurements may be easier to detect, and more likely tocause interference, than others. This may particularly be the case inembodiments with emitter modules containing emission LED elementsemitting different colors of light. Certain combinations of LED elementand detector may result in significantly higher photocurrent signals.Measurements using these emitter/detector combinations may be referredto as “beacon” measurements. The magnitude of the photocurrent signalfor a particular measurement depends on factors including the luminousflux emitted by the LED element, the sensitivity of the detector, andhow well the emitter and detector are matched in terms of spectralresponse. As an example, one measurement for a multi-color emissionmodule that may result in a relatively high photocurrent signal ismeasurement of a green emission LED element using a detector configuredto detect red light (in an embodiment, the detector is a red LEDconfigured as a detector).

For the example described above of an emitter module having 12compensation measurements including 6 photocurrent measurements,consider an embodiment in which two of the photocurrent measurementsresult in significantly higher photocurrent signals than the otherphotocurrent measurements. In such an embodiment, the number ofdetection intervals used before starting compensation measurements maybe chosen such that one of these higher-photocurrent signals would beexpected to occur if a nearby device is performing compensationmeasurements. If the sequence of the measurements is not known, forexample, 11 intervals without detection of a non-constant illuminationwould be needed to be certain that one of the 2 “beacon” measurementsshould have occurred if interfering measurements are in progress.Alternatively, if the 2 “beacon” measurements are known to be evenlyspaced within the measurement sequence (6 measurements apart, in thisexample), 6 intervals without detection of a non-constant illuminationwould be sufficient before beginning compensation measurements.

The embodiments described above relating to determining a number ofdetection intervals to use before starting compensation measurements canbe illustrated using a timing diagram such as that of FIG. 13A. In FIG.13A, detection intervals 1310 are used to determine whether measurementstaken by another lamp can be detected. If no other measurements aredetected, compensation measurements are initiated during subsequentintervals denoted in FIG. 13A as measurement intervals 1320. Thenecessary number of detection intervals 1310 in which no interferingmeasurement is detected depends on factors such as the number, natureand sequencing of compensation measurements, as discussed further above.The specific measurements illustrated in FIG. 13A as being performedduring the first of measurement intervals 1320 are merely exemplary.

An alternative approach to that of FIG. 13A is shown in FIG. 13B. In thetiming diagram of FIG. 13B, detection intervals 1310 are alternated withintervals in which non-sensitive measurements 1322 are taken.Non-sensitive measurements as used herein are measurements not affectedsignificantly by external illumination. In an embodiment, non-sensitivemeasurements include forward voltage measurements across an LED elementor a photodetector. As discussed further above in connection with FIG.12, such forward voltage measurements are expected to be non-sensitiveif the forward-bias induced current in the measured LED element is largecompared to the photocurrent induced by the external illumination. Atiming sequence such as that of FIG. 13B may allow non-sensitivemeasurements to be taken earlier, while it is still being determinedwhether measurements sensitive to interfering illumination (denoted assensitive measurements 1324) can be taken without interference. In anembodiment, detection for interfering measurements may be performedduring the same interval as one of non-sensitive measurements 1322, aslong as the detector used for detecting interference is not involved inthe non-sensitive measurement. In an embodiment for which thenon-sensitive measurement is a forward voltage measurement, the forwardvoltage measurement would need to be performed at a non-illuminatinglevel of drive current to avoid error in performing detection at thesame time.

In an embodiment for which non-sensitive measurements are performedduring an overall detection sequence but detection is not performedduring the intervals in which non-sensitive measurements are taken, theexpected measurement sequence of any interfering devices would need toinclude enough consecutive higher-intensity measurements that ameasurement sequence performed by a nearby device would be detectedduring one of the intervals when detection is performed. For example, inan embodiment of FIG. 13B in which no detection is performed during oneor both of the intervals allocated to non-sensitive measurements 1322,higher-intensity measurements performed by an interfering device wouldneed to be grouped so that at least two of the high-intensitymeasurements are performed in consecutive intervals. In this way, if theinterfering device is performing measurements and one high-intensitymeasurement occurs in the same interval as a non-sensitive measurement1322 and is not detected, the other consecutive high-intensitymeasurement would be detected during either the preceding or succeedingdetection interval 1310.

The timing diagrams of FIGS. 13A and 13B illustrate examples of anapproach in which some number of detection intervals is used to obtainan indication that no nearby device is performing interferingmeasurements. When no interfering measurement is observed after asufficient number of detection intervals, compensation measurements areinitiated during subsequent intervals. If, on the other hand, anon-constant illumination is detected during a detection interval, thisis an indication that a nearby device is performing interferingmeasurements. Detection of a constant illumination during the intervalis not associated with an interfering measurement in such an embodiment,because the effects of a constant external illumination on aphotocurrent measurement can be removed by ambient subtraction such asthat illustrated in FIG. 11A. In some embodiments, detection can beperformed by taking photocurrent measurements during each of twoportions of the interval, and then subtracting the photocurrents, in themanner described above for FIG. 11A. A non-zero result of thesubtraction in such an embodiment indicates a non-constant illuminationduring the interval.

In an embodiment, detection of a non-constant illumination during adetection interval causes an illumination device to discontinue thedetection sequence and return to driving the emission LED elements inthe device to provide continuous illumination. In such an embodiment,the illumination device may be returned to a continuous illuminationstate uninterrupted by detection intervals or measurement intervals,similar to illumination periods 1010 of FIG. 10 above. In an alternativeembodiment, a sequence of alternating illumination periods and intervalswith the emission LED elements turned to non-operative levels may becontinued after the detection sequence is discontinued, but withoutmeasurement taking place during the intervals. In a further embodiment,any intervals present after the detection sequence is suspended wouldnot be used for detection or measurement until such time that adetection sequence is restarted.

When the detection sequence is discontinued after detection of anon-constant illumination during a detection interval, the measurementcontrol circuit of the illumination device waits, in one embodiment, forsome delay time before restarting the detection sequence. In a furtherembodiment, the delay time is a randomized delay time. After waiting forthe delay time, the measurement control circuit may in one embodimentstart again at the beginning of the detection sequence that was abortedupon detection of the non-continuous illumination. Alternatively, insome embodiments the detection sequence may be picked up at a pointafter the beginning of the sequence. In an embodiment, the detectionsequence is started again at the point in the sequence when thenon-continuous illumination was previously detected. Such an embodimentmay be suitable, for example, in a sequence such as that of FIG. 13B inwhich some non-sensitive measurements are performed successfully in anearlier detection sequence before it is aborted.

As an alternative to the above-described embodiments of suspending adetection sequence and resuming detection after a delay, anotherapproach to handling detection of a non-constant illumination during adetection interval may be suitable in certain embodiments. In anembodiment for which the sequence of measurements expected to beperformed by an interfering device is known, detection of a non-constantillumination during one or more detection intervals may allow ameasurement control circuit to predict which upcoming intervals will orwill not contain interfering measurements. In such an embodiment, themeasurement control circuit may be able to select a starting intervalfor its own measurement sequence such that each of the two devices isable to complete its respective measurement sequence without obtainingerroneous results. An example of such a scenario is illustrated by FIG.14.

The pair of timing diagrams in FIG. 14 is for two emitter modules,designated Lamp A and Lamp B, similar to those described in thediscussion of FIG. 12 above. Each lamp is operating in a compensationmode such as that within a compensation period 1020 of FIG. 10, in whichperiods of illumination 1402 are interrupted by intervals includingintervals 1410, 1420, 1430, 1440 and 1450. At the beginning of eachinterval the emission LED elements within the lamp are turned off (or toa non-illuminating level) and detection may be performed or ameasurement associated with a particular LED element and/or detector maybe taken. In the embodiment of FIG. 14, intervals 1410 and 1420 aredetection intervals for Lamp A. These intervals are measurementintervals for Lamp B, however. In the embodiment of FIG. 14, Lamp B iscarrying out a sequence of 8 measurements in which a forward voltage foreach of four emission LED elements is followed by a measurement ofphotocurrent induced in a detector when a drive current is applied tothat LED element. The lower timing diagram in FIG. 14 therefore showsthe entire sequence of measurements carried out by Lamp B. In anembodiment, this measurement sequence is repeated continuously usingsubsequent intervals. In another embodiment, the lamp returns to acontinuous illumination mode such as an illumination period 1010 of FIG.10, and the measurement sequence is repeated if a change in operatingconditions is detected or at certain preset times.

During interval 1410, Lamp B carries out a forward voltage measurementV_(f1B) of a first emission LED element. Even in an embodiment for whichLamps A and B are in close proximity and/or facing one another, Lamp Adoes not detect any significant non-constant illumination from themeasurement by Lamp B as long as the drive current for the measurementV_(f1B) is at a level too low to result in illumination. During interval1420, however, Lamp A does, in this embodiment, detect a non-constantillumination associated with the measurement by Lamp B of photocurrentI_(ph1B) induced in a detector when the first LED element isilluminated. In the embodiment of FIG. 14, the sequence of measurementsemployed by potentially interfering lamps, including Lamp B, is known tothe control circuit of Lamp A, and Lamp A employs the same sequence forits own compensation measurements. Upon detecting a non-constantillumination during interval 1420, the control circuit of Lamp Adetermines that an interfering lamp made a photocurrent measurementduring that interval. Because the measurement sequence is known toalternate photocurrent measurements with non-illuminating forwardvoltage measurements, the control circuit of Lamp A can predict that theinterfering lamp will make a forward voltage measurement during the nextinterval, interval 1430. Because the measurement sequence begins with aforward voltage measurement, the control circuit of Lamp A waits for oneadditional interval and begins the measurement sequence for Lamp A atinterval 1440. In this way, the photocurrent measurements by Lamp B lineup in the same intervals as the non-sensitive, and non-interfering,forward voltage measurements by Lamp A.

In the embodiment of FIG. 14, both Lamps A and B can keep repeating themeasurement sequence continuously in subsequent intervals, if desired,without interfering with each other's measurements. An approach such asthat of FIG. 14, in which potentially interfering lamps performmeasurement sequences in an overlapping manner that avoids interference,may be particularly suitable for embodiments in which a measurementsequence is repeated continuously. In an embodiment with continuouscompensation measurements, the alternate approach described above, ofsuspending measurements when an interference is detected and attemptingmeasurements again after a delay, may be less effective. For themeasurement sequence used in FIG. 14 having alternating photocurrent andforward voltage measurements, the control circuit of Lamp A candetermine an interval for starting a non-interfering measurementsequence after detection of just one interfering measurement. Inembodiments using different measurement sequences, the control circuitmay need to detect multiple interfering measurements in order todetermine a starting interval for a non-interfering measurementsequence. In the case of some measurement sequences, overlapping butnon-interfering measurement sequences may not be available.

The approach of FIG. 14 depends on access by the control circuit of anillumination device to the measurement sequence used by potentialinterfering devices. One embodiment in which the control circuit mayhave such information is an installation in which the lamps in closeproximity to one another are all made by the same manufacturer and usethe same control sequence. In another embodiment, a control circuit hasinformation on measurement sequences of potential interfering lampsbecause the lamps in close proximity to one another are manufactured toa common standard that specifies the measurement sequence. Ininstallations having lamps in close proximity that use differentmeasurement sequences, information regarding the measurement sequencesof various other lamps may in some embodiments be available to thecontrol circuit of an illumination device. An illumination device may incertain embodiments include a data structure storing configurationinformation including compensation measurement sequences for variouspotentially interfering lamp models. In embodiments for whichinterference by lamps having multiple different measurement sequences isa possibility, the control circuit may need to detect multipleinterfering measurements before determining which measurement sequenceis being used by another device and whether overlapping measurementsequences are possible without interference.

The discussion above of FIGS. 13 and 14 describes ways that detectionduring some number of intervals before performing compensationmeasurements during subsequent intervals can help to avoid measurementerrors caused by interfering measurements by nearby illuminationdevices. In some cases, however, measurement errors may occur despiteuse of the above-described detection techniques. For example, aprediction that a lamp may safely begin making measurements based on theexpected measurement sequence of a single interfering lamp may be inerror if multiple nearby lamps are making measurements. As anotherexample, measurement errors can occur if two or more lamps areperforming detection during the same intervals and, each detecting noother measurements, both begin measurements at the same time.

In an embodiment, measurement errors are detected by checking to seewhether a measured value is within an expected range. In a furtherembodiment, the expected range is based on the most recently storedvalue of the measured quantity. In such an embodiment, the expectedrange accounts for the magnitude of expected variations in the measuredquantity caused by factors such as LED aging or temperature change of anLED element. In one embodiment, a measured value is outside of theexpected range if it varies by more than about 5 percent from the mostrecently stored value of the measured quantity. In another embodiment, ameasured value is outside of the expected range if it varies by morethan about 3 percent from the most recently stored value. In yet anotherembodiment, a measured value is outside of the expected range if itvaries by more than about 2 percent from the most recently stored value.Other thresholds for considering a measurement out-of-range may be used,depending on factors such as the volatility of the particular quantitybeing measured and the degree of accuracy required for compensation andcontrol of the illumination device. If the measured value is outside ofthe expected range, the measured value is discarded rather than stored.In an embodiment, the measurement sequence continues after an out-ofrange measurement is detected, with in-range measurements stored whileout-of-range measurements are discarded. In an alternative embodiment,an out-of-range measurement causes the measurement sequence to besuspended. In such an embodiment, the control circuit of theillumination device may wait for a delay time and then attempt themeasurement sequence again. The new attempt may start at the beginningof the sequence, or alternatively may start with the measurement thatwas out of range. In another embodiment in which the measurementsequence is suspended after an out-of-range measurement, the controlcircuit may wait for a delay time and then begin a detection sequencebefore attempting measurements again.

Checking for whether a measurement is in range is in some embodimentscombined with methods described above for detection during some numberof intervals before performing compensation measurements. In analternative embodiment, measurements are performed without any detectionintervals beforehand, with the measured values checked for being out ofan expected range. In still another embodiment, measurements areinitially performed without detection beforehand, but if an out-of-rangevalue is obtained, a detection method as described above is employedbefore resuming measurements. In some embodiments, checking for whethera measurement is in range is performed only for interference-sensitivemeasurements such as photocurrent measurements. In other embodiments,all measured values are checked for being within an expected range.

Approaches described above to avoiding interference from nearbyillumination devices when performing compensation measurements includeperforming detection to predict interference-free intervals for takingmeasurements, checking measured values to determine whether measurementerror has occurred, and suspending and reattempting detection and/ormeasurements in the event that interference is detected. Anotherapproach to avoiding interference is to use a different set of intervalsthan that used by a potentially interfering device. In an embodiment ofthis approach, one set of periodic intervals is established having afirst offset time from a periodic timing reference, while another set ofperiodic intervals is established having a second offset time from thetiming reference. An exemplary timing diagram illustrating such anembodiment is shown in FIG. 15.

In the embodiment of FIG. 15, a timing reference signal 1520 isgenerated from an AC reference signal 1510. In an embodiment, timingreference signal 1520 is generated from AC signal 1510 using a phaselocked loop (PLL) circuit. In the example of FIG. 15, reference signal1520 has a frequency of six times that of AC signal 1510. In anembodiment, AC signal 1510 is the AC mains signal, typically having afrequency of 50 Hz or 60 Hz. For an AC mains frequency of 60 Hz,reference signal 1520 has a frequency of 360 Hz in the embodiment ofFIG. 15. Waveform 1530 illustrates the drive current variation with timefor an illumination device, such as an emitter module, using a first setof intervals for compensation measurements. As discussed in connectionwith FIG. 6 above, “on” current I_(on) represents a combination of oneor more different drive currents applied as appropriate to respectivedifferent LED elements within the illumination device, to produce thedesired illumination. During periodic measurement intervals the drivecurrents are reduced to a level I_(off) at which none of the LEDelements are operating, or illuminated, except for a single LED elementthat may be subject to measurement during the interval. Each of theintervals has a duration 1532 and is separated from a rising edge oftiming reference 1520 by a first offset 1536. Waveform 1540 illustratesthe drive current variation with time for an illumination device using asecond set of intervals for compensation measurements. Waveform 1540 issimilar to waveform 1530, except that the periodic intervals in waveform1540 are separated from a rising edge of timing reference 1520 by asecond offset 1546.

If one emitter module is configured to perform compensation measurementsusing a first set of measurement intervals such as those of waveform1530, and another emitter module is configured to perform itscompensation measurements using a second set of measurement intervalssuch as those of waveform 1540, measurements by the two emitter moduleswill not interfere with one another because the two sets of measurementintervals are displaced in time. In an embodiment, lamps or emittermodules that are to be placed in close proximity are assigned todifferent sets of measurement intervals. Such an embodiment may beparticularly suitable for illumination fixtures containing multiplelamps or emitter modules. In another embodiment, an emitter module mayinitially use one set of measurement intervals and later switch toanother set of measurement intervals if interference from nearby devicesis encountered. This type of embodiment may be suitable in the case ofan individual emitter module, since the configuration of lamps that itmay be operated in proximity to is typically not known.

In the example described above of a 60 Hz AC signal and a 360 Hz timingreference signal used in the embodiment of FIG. 15, timing referencesignal 1520 has a period of approximately 2.8 milliseconds. Using thesevalues and the dimensions as drawn in FIG. 15, the measurement intervalsof waveforms 1530 and 1540 have a duration of approximately 550microseconds while the first offset is approximately 800 microsecondsand the second offset approximately 2 milliseconds. It should be notedthat the measurement intervals may have any duration sufficient toperform any compensation measurement needed. In an embodiment, themeasurement interval should be long enough to allow a period ofmeasuring the desired quantity and a period for ambient measurement. Atthe same time, it is preferred in some embodiments to have measurementintervals be as short as possible in order to reduce effects such as“flicker” caused by turning the LED elements on and off. In oneembodiment, the measurement interval duration is approximately 100microseconds. The number of different sets of measurement intervals thatmay be used depends on the period of the timing reference signal and theduration of the measurement interval.

In one embodiment having a timing reference signal with frequency of aninteger N times the frequency of an AC reference signal (like theembodiment of FIG. 15, where N=6), the number of intervals in ameasurement sequence is set to be an integral multiple of N. For theexample of FIG. 15 in which N=6, the number of intervals in themeasurement sequence in this embodiment would be set to a multiple of 6,even if some intervals were left empty in order to do so. In this way,repetition of the measurement sequence would cause repetitions of anyindividual measurement to occur at the same point in the phase of the ACsignal. In an alternative embodiment with a timing reference signalhaving a frequency of N times the AC reference signal, the number ofintervals in the measurement sequence is instead set to a number that isnot an integral multiple of N. In such an embodiment repetition of themeasurement sequence would cause repetitions of any individualmeasurement to occur at different points in the phase of the AC signal.In a further embodiment, values obtained from repetitions of anindividual measurement are averaged. In such an embodiment, use of anumber of measurements that is not an integral multiple of N may providea more accurate measurement when results from repetitions of ameasurement taken at different AC phase points are averaged.

Flowcharts of exemplary methods of performing interference-resistantcompensation measurements using the approaches described above are shownin FIGS. 16A through 16C. The flowchart of FIG. 16A is for a method inwhich no detection is performed before beginning a sequence ofmeasurements. In the embodiment of FIG. 16A, photocurrent measurementsinclude subtraction of ambient photocurrent, and the method includesdetermining whether photocurrent values are within an expected range.The starting point for the method is operation of one or more emissionLED elements within an illumination device or emitter module atrespective drive currents to produce the desired illumination (step1602). This illumination is continued until the control circuit of theillumination device determines that it is time to take compensationmeasurements (decision 1604). In some embodiments, compensationmeasurements are performed at specific times. In other embodiments themeasurements may be performed when a change is detected in operatingconditions, such as temperature of the illumination device or a changein drive current supplied to one or more of the emission LEDs to alterthe lumen output or color point setting of the illumination device. Instill other embodiments, periodic compensation measurement intervals maybe created throughout the time the illumination fixture is operating,and compensation measurement sequences may be continually repeated usingthose intervals.

In the embodiment of FIG. 16A, a measurement counter is initialized tokeep track of which measurements in a measurement sequence have beenperformed (step 1606). All of the emission LED elements are then turnedoff (to non-operative or non-illuminating levels) at the start of thenext measurement interval (step 1608). The measurement interval is oneof a set of intervals such as those discussed in connection with FIGS.6, 8 and 10-15 above. If the measurement to be performed is not aphotocurrent measurement, the measurement is performed during theinterval and the result of the measurement is stored (decision 1610,step 1612, step 1614). A non-photocurrent measurement may include, forexample, a forward voltage measurement across an emission LED or aphotodetector. Methods of performing forward voltage measurements aredescribed further in the co-pending applications referenced herein.After the result is stored, the measurement counter is incremented andthe emission LED elements are turned back on to produce illumination(steps 1616, 1618).

If a photocurrent measurement is performed, the emission LED element tobe tested is turned on using the desired drive current during a firstpart of the measurement interval (decision 1610 and step 1622). In oneembodiment, the emission LED element is turned on for half of themeasurement interval. In other embodiments, the emission LED element isturned on for a different fraction of the measurement interval. Thephotocurrent on a detector within the illumination device or emittermodule is measured during the part of the measurement interval when thetested LED element is turned on (step 1624). The detector used in themeasurement may be referred to herein as a measurement photodetector andthe photocurrent detected by the measurement may be referred to as ameasurement photocurrent. During a second part of the measurementinterval, the tested LED element is turned off (while the other emissionLED elements remain turned off) (step 1626). The ambient or backgroundphotocurrent induced in the detector is measured during this second partof the measurement interval (step 1628). As noted in the discussion ofFIG. 11 above, the photocurrent values may be obtained using averagingand/or other signal processing techniques known to those of ordinaryskill in the art in view of this disclosure. In some embodiments, thefirst part of the measurement interval during which the LED element isturned on is at the beginning of the interval, as illustrated by portion1104 of FIG. 11. In other embodiments, the first part is at the end ofthe interval, and the ambient measurement in the second part of theinterval is done before the measurement of photocurrent from the drivenLED element.

When both the photocurrent induced by the driven LED element and theambient photocurrent have been measured, the ambient photocurrent issubtracted from the photocurrent induced by the driven emission LEDelement to obtain a corrected photocurrent (step 1630). In anembodiment, this subtraction is done in hardware. The correctedphotocurrent is then checked to see whether it is within an expectedrange (decision 1632). In an embodiment, the expected range is based ona target value of the photocurrent, or on the most recent reliablemeasured value. The expected range is in some embodiments set to belarger than the expected variation of the photocurrent caused bytemperature variation or LED aging. If the corrected photocurrent iswithin the expected range, it is stored (step 1614) and the measurementcounter is incremented (step 1616).

In the embodiment of FIG. 16A, if the corrected photocurrent is out ofthe expected range, storage of the corrected value is skipped (N branchof decision 1632). Incrementing of the measurement counter andcontinuing on with the next measurement in the sequence (steps 1616 and1618, decision 1620) are performed in the same way whether thephotocurrent measurement is stored or discarded. In this embodiment, ameasurement for which the result is not stored can be attempted againwhen its turn comes up in the next measurement sequence. In an alternateembodiment to that of FIG. 16A, the measurement sequence is suspendedwhen an out-of-range measurement is discovered. In such an embodiment,the measurement sequence may be re-attempted after a delay time or afterchanging to a different set of measurement intervals. Some of theseoptions are illustrated in the method of FIG. 16B discussed below.

At the end of the measurement interval, one or more of the emission LEDelements are again operated to produce the desired illumination (step1618). As compensation measurements are taken and evaluated, the drivecurrents applied to the respective LED elements to obtain desiredillumination may be adjusted, as described further in the co-pendingapplications referenced herein. In the embodiment of FIG. 16A, thesequence of measurements is continued, with any photocurrentmeasurements either stored or discarded, until the end of the sequence(decision 1620). At the end of the sequence, a new measurement sequencemay be started as determined by the control circuit (decision 1604). Asdiscussed above, measurement sequences may be repeated continually insome embodiments, or performed only at certain times or under certainconditions. In one embodiment, a measurement sequence is repeated if anout-of-range measurement is detected in the previous sequence.

Variations of the method of FIG. 16A will be recognized by one ofordinary skill in the art in view of this disclosure. For example, forthis and all flowcharts described herein, a group of steps in betweentwo decision points of the flowchart may often be performed in more thanone order. Although the embodiment of FIG. 16A performs ambientsubtraction only for photocurrent measurements, in another embodiment asimilar scheme of interval portions and subtraction could be used fornon-photocurrent measurements. In some embodiments, non-photocurrentmeasurements can also be checked for being within an expected range.

An exemplary flowchart for a method of detecting during a series ofintervals prior to starting compensation measurements is shown in FIG.16B. In the same manner as discussed above for FIG. 16A, the methodbegins with operation of one or more emission LED elements to producethe desired illumination (step 1602). This illumination is continueduntil the control circuit of the illumination device determines that itis time to take compensation measurements (decision 1604). After it isdetermined that compensation measurements are to be taken, the controlcircuit initializes a counter for “collisions,” or determinations thatanother device is making a measurement during an interval. Counters arealso initialized for free intervals, or intervals in which nomeasurement by another device is detected, and for contiguous freeintervals since the last collision (step 1634). All of the emission LEDelements are turned “off”, or to non-operative levels, at the start ofthe next interval (step 1636), which in the embodiment of FIG. 16B isused as a detection interval similar to intervals 1310 in FIG. 13. Thephotocurrent induced in a detector within the illumination device ismonitored during the detection interval (step 1638). The detector usedduring a detection interval may be referred to herein as a “detectioninterval photodetector,” and the photocurrent induced during thedetection interval as “detection photocurrent.” In an embodiment, thedetection interval photodetector and measurement photodetector usedduring compensation measurements are the same photodetector. In analternative embodiment, the detection interval photodetector andmeasurement photodetectors are different detectors. In some embodiments,different measurement photodetectors are used for photocurrentmeasurements of different LED elements. Such embodiments may allow amore favorable combination of wavelengths of the tested LED element andthe photodetector. Unless otherwise specified, any of the detectorsreferenced herein may be either a dedicated photodetector or an LEDelement temporarily configured as a photodetector.

If no non-constant illumination is detected during the interval(decisions 1640 and 1654), a “free” interval is recorded by incrementingthe free interval counter and contiguous free interval counter (step1658). The emission LED elements are turned back on to resumeillumination at the end of the interval (step 1656). In the embodimentof FIG. 16B, a number of contiguous free intervals has been designatedas an indicator that no other device is likely to be taking measurementsusing the same set of intervals. Considerations for determining asuitable number of free contiguous intervals are described above in thediscussion of FIGS. 12 and 13. When the designated number of contiguousfree intervals has been reached, compensation measurements are startedin the next interval (decision 1660 and step 1662). Measurements maythen proceed in any suitable manner, including a manner similar to thatillustrated in FIG. 16A.

If non-constant illumination is detected during an interval, thecollision counter is incremented and the contiguous free intervalcounter is reset (decision 1640 and steps 1644 and 1646). The emissionLED elements are turned back on as usual to resume illumination at theend of the interval (step 1642). If a maximum number of collisions hasnot been reached, the control circuit waits for a delay time beforeattempting detection again (decision 1648, steps 1650 and 1636). In anembodiment, the delay time is a randomized delay time. In a furtherembodiment, the delay time is determined using the collision counter,such that after each successive collision the delay time isprogressively longer. For example, in one embodiment the delay time israndomized within a specific range, and that range is set toprogressively higher values after each successive collision. In afurther embodiment, the delay time increases after each successivecollision at an exponential rate.

In an embodiment of the method of FIG. 16B, detection of non-constantillumination refers to detection of illumination having an intensitythat varies substantially with time during the detection interval, orduring a portion of the detection interval in which detection isperformed. In a further embodiment, illumination intensity variessubstantially with time if the variation would be large enough to inducea significant error in a photocurrent measurement conducted during thesame interval. In some embodiments, a substantial variation in intensityis defined in terms of the intensity of illumination produced by aphotocurrent measurement within the illumination device performing amethod such as that of FIG. 16B. In a further embodiment, a substantialvariation in intensity is defined in terms of the intensity ofillumination produced by the LED element within the illumination deviceproducing the lowest illumination intensity during photocurrentmeasurements performed as part of a compensation measurement sequence.For example, a substantial variation in intensity with time may bedefined in one embodiment as a variation large enough that the change inintensity during the interval is greater than about 5% of the intensityproduced by the LED element within the illumination device having thelowest illumination intensity during photocurrent measurements. In afurther embodiment, a substantial variation is a variation large enoughthat the change in intensity during the interval is greater than about3% of the intensity produced by the LED element within the illuminationdevice having the lowest illumination intensity during photocurrentmeasurements. In a still further embodiment, a substantial variation isa variation large enough that the change in intensity during theinterval is greater than about 2% of the intensity produced by the LEDelement within the illumination device having the lowest illuminationintensity during photocurrent measurements. Other thresholds fordetecting interference may be used, depending on factors such as thedegree of accuracy required for compensation and control of theillumination device.

If measurements by other devices continue to be detected during repeatedattempts separated by delay times, a maximum number of collisions may bereached (decision 1648). At this point, the control circuit changes to adifferent series of measurement intervals, separated from a timingreference by a different offset time (step 1652). Such sets of intervalsare described above in connection with waveforms 1530 and 1540 in FIG.15. In the embodiment of FIG. 16B, the detection sequence is restartedby resetting all counters after a change to a new set of intervals (step1634). A change to a new series of intervals such as that of FIG. 16Bmay be particularly suitable in the case of an illumination deviceincluding a single lamp or emission module. Changing of an intervalseries may be less appropriate in the case of a multiple-lamp device,such as that described below in connection with FIG. 18. In a multi-lampdevice, each lamp may be assigned to a specific interval series in orderto avoid interference between them, such that changing of the intervalseries could in some cases increase the likelihood of interference.

Variations of the method of FIG. 16B will be recognized by one ofordinary skill in the art in view of this disclosure. For example, inthe embodiment of FIG. 16B a collision is detected by monitoring theentire detection interval for non-constant illumination. In anotherembodiment, only a portion of the detection interval is monitored, basedon knowledge of when during the interval a change in illuminationintensity caused by an interfering measurement is expected to takeplace. For example, the expected intensity variation may be associatedwith a transition between driving an LED element for a photocurrentmeasurement and having the LED element turned off for an ambientphotocurrent measurement, as shown in FIG. 11A. In such an embodiment,if the time of the change between the LED measurement and ambientmeasurement portions of the interval is known, the monitoring can bedone over a range including that transition time.

An alternative method of detecting prior to starting compensationmeasurements is illustrated by the flowchart of FIG. 16C. The method ofFIG. 16C is similar in some respects to that of FIG. 16B, but in FIG.16C there does not always have to be a certain number of contiguous freeintervals detected before compensation measurements can start. Incertain situations the method of FIG. 16C allows a measurement sequenceto be started if it can be overlapped with an ongoing measurementsequence of another device in such a way that the measurements do notinterfere with (i.e. cause measurement errors for) one another.

Although not shown in FIG. 16C, the context of the method is the same asfor FIGS. 16A and 16B in that one or more LED elements are operated toproduce the desired illumination until the control circuit of theillumination device determines that it is time to take compensationmeasurements (see steps 1602 and 1604 of FIGS. 16A and 16B). Monitoringfor non-constant illumination is performed in the same manner as forFIG. 16B, and in the event that a designated number of contiguous freeintervals is reached, a measurement sequence is started in the same wayas in the method of FIG. 16B (steps 1638-1662, going down right side offlowchart). The method of FIG. 16C differs from that of FIG. 16B in theevent that a collision is detected, however. Instead of automaticallyinstituting a delay or a change in interval series after a collision isdetected, the control circuit in the embodiment of FIG. 16C determineswhether the measurement sequence causing the detected collision is known(decision 1664). If the interfering measurement sequence is known, thecontrol circuit determines whether it can initiate compensationmeasurements that overlap with those of the other device in a mannerthat avoids interference (step 1670).

In an embodiment, determinations as to whether an interferingmeasurement sequence is known and whether overlapping, butnon-interfering, measurements may be conducted are done usingconfiguration information such as that shown in FIG. 17. The chart ofFIG. 17 includes exemplary configuration information that may becontained in a data structure stored on the illumination device. In anembodiment, such configuration information may be stored in the samestorage medium that contains a calibration table used for compensatingthe operation of the illumination device to account for changes intemperature or LED characteristics. In the embodiment of FIG. 17,configuration information 1700 includes measurement sequences for threedifferent illumination devices, designated Brand A, Brand B, and BrandC. In an embodiment, the three illumination devices are made bydifferent manufacturers. Configuration information 1702 is for the BrandA device, while information 1704 and 1706 is for the Brand B and Brand Cdevices, respectively. Controlled device information 1710 indicates thatthe controlled device (the one that configuration information 1700 isstored in) is a Brand A device in this embodiment.

Sequence information 1708 includes the sequence of compensationmeasurements performed for each device. In the embodiment of FIG. 17sequence information 1708 includes the specific measurement performed ineach interval of the sequence, as well as whether the measurement isSensitive or Non-sensitive (to external illumination) and whether themeasurement is Interfering or Non-interfering. In this embodiment,photocurrent measurements are all considered to be both sensitive andinterfering, since photocurrent measurements both detect illumination(and are therefore sensitive to external illumination) and createillumination from the tested LED element (and therefore can interferewith another photocurrent measurement). In this embodiment, forwardvoltage measurements, whether across an emission LED element (e.g.V_(f1)) or a detector (e.g. V_(fd1)), are considered to be non-sensitiveand non-interfering. That a forward voltage measurement isnon-interfering is believed to be a suitable assumption when the forwardvoltage measurements are performed with low drive current levels so thatthe measured devices do not produce illumination. In other embodimentswith higher drive current levels, a forward voltage measurement may bean interfering measurement (though probably still not a sensitivemeasurement). As discussed further above with reference to FIGS. 12 and13, a forward voltage measurement can be considered non-sensitive if theforward bias induced current in the measured LED element is large withrespect to any photocurrent induced by external illumination. In theembodiment of FIG. 17, the measurement sequence for each device includestwo empty intervals to bring the length of the sequence to 12 intervals.Such empty intervals are non-sensitive and non-interfering. The 12interval length of the measurement sequences in FIG. 17 is merelyexemplary. Any number of intervals may be used to form a measurementsequence, and a set of measurement sequences included in configurationinformation such as configuration information 1700 may include sequenceshaving different lengths (i.e., including different numbers ofmeasurement intervals).

In the embodiment of FIG. 17, actual measurement sequences for all threedevices are known. In other embodiments, specific measurement sequencesfor devices made by other manufacturers may not be known. In such anembodiment, data on whether measurements are sensitive or interferingmay be experimentally obtainable (for example, through use of anexternal detector), even if the actual measurements are unknown. In analternative embodiment of the method of FIG. 16C, decision block 1664determines whether the order of interfering and non-interferingmeasurements within the interfering measurement sequence is known,rather than whether the actual measurements within the sequence areknown.

The remaining information in configuration data 1700 characterizes themeasurement sequence for each device in ways that may be helpful indetermining whether an overlapping measurement sequence can be formed.In an embodiment, an overlapping but not interfering measurementsequence can be conducted as long as any sensitive measurements in onesequence of measurements performed by one device are not performed inthe same interval as an interfering measurement in another sequence ofmeasurements performed by a nearby device. Because in the embodiment ofFIG. 17 sensitive measurements and interfering measurements are thesame, much of the configuration information is described in terms ofsensitive measurements, but is also applicable to interferingmeasurements. In this embodiment, the rule for conducting overlappingbut not interfering measurements can be restated as making sure that asensitive measurement in one sequence is not performed in the sameinterval as a sensitive measurement in the other sequence.

Within configuration information 1700, number of sensitive measurements1712 indicates the number of sensitive measurements within eachsequence. In the embodiment of FIG. 17 there are four sensitivemeasurements (the four photocurrent measurements) in each sequence. Thenumber of non-sensitive measurements 1714 is accordingly eight for eachof the devices. As a first-order indicator, a high fraction of sensitive(or interfering) measurements in a measurement sequence can make it lesslikely that an overlapping measurement sequence can be performed. Forexample, if in an alternate embodiment the measurement sequence for theBrand A device had 7 out of 12 interfering measurements rather than 4out of 12, it would be very difficult to overlap measurement sequencesfor two Brand A devices in close proximity to one another without havinga sensitive measurement by one device performed in the same interval asa sensitive (and interfering) measurement by the other device. It couldbe done if each device ran its measurement sequence only once withoutrepeating, and most of the sensitive measurements by one device werefinished before the second device started its sequence. Anon-interfering overlap would not be possible in this embodiment,however, if either of the devices were configured to immediately repeatits measurement sequence.

Same-sequence non-interfering offset 1716 refers to a number ofintervals by which a device performing a measurement sequence needs tooffset (i.e., delay) its sequence with respect to another deviceperforming the same sequence. For example, if a Brand A device detecteda photocurrent measurement performed by an interfering device and it wasknown that the interfering device was also a Brand A device, it would beknown from Brand A configuration information 1702 that the nextmeasurement, if any, by the interfering device would be anon-interfering (non-photocurrent) measurement. The detecting devicecould not start its measurement sequence during that next interval,because the non-interfering first measurement of its sequence wouldalign with the non-interfering next measurement of the interferingsequence. Because much of the Brand A measurement sequence alternatesbetween interfering and non-interfering measurements, aligning twonon-interfering measurements between the devices would likely causealignment of two interfering (and sensitive) measurements in asubsequent interval of the sequence. If the detecting device delays onemore interval before starting its sequence, however, any remainingsensitive (photocurrent) measurements by the interfering device shouldalign with a non-sensitive measurement by the detecting device. Thisdelay has the effect of offsetting, or shifting, the measurementsequence of the detecting device by an odd number of intervals from thatof the interfering device.

Using a similar analysis for the measurement sequence of the Brand Bdevice, it can be seen from configuration information 1704 that anoffset 1716 of either 2 or 6 intervals would allow another Brand Bdevice to perform an overlapping measurement sequence. Similarly, forthe sequence of the Brand C device, an offset of between 4 and 8intervals would allow another Brand C device to perform an overlappingbut non-interfering measurement sequence.

Another quantity included in configuration information 1700 is intervalrange 1718 including all sensitive measurements. The Brand A sequencehas a range 1718 of 7 intervals, from interval 2 to interval 8, in whichall of the sensitive measurements are performed. The Brand B sequencehas a range 1718 of 6 intervals, from interval 3 to interval 8. For thebrand C device, all of the sensitive measurements are performed within arange 1718 of 4 intervals.

Also included in configuration information 1700 is interval range 1720of the most contiguous non-sensitive measurements within a measurementsequence. Interval range 1720 is 5 for the sequence of Brand A, frominterval 9 to interval 1 (assuming that the measurement sequence iscontinually repeated). For the measurement sequence of Brand B, intervalrange 1720 is 6 intervals, from interval 9 to interval 2. For thesequence of Brand C, interval range 1720 is eight intervals, frominterval 5 to interval 12. Interval ranges 1718 and 1720 may be usefulin determining whether different measurement sequences, such as thoseused by different device manufacturers, may be overlapped withoutinterference. For example, the measurement sequences of the threedevices of configuration information 1700 are too different to allownon-interfering overlap of two different device sequences using a simpleone- or two-interval shift. In some cases, however, a larger shift canalign a contiguous range of non-sensitive measurements in one sequencewith the entire range of sensitive measurements in another sequence. Toillustrate, the measurement sequence of Brand A in FIG. 17 can overlapwith the sequence of Brand C if the Brand A sequence is shifted so thatinterval 2 of the Brand A sequence is aligned with interval 5 or 6 ofthe Brand C sequence. In this way, all of the sensitive measurements inthe Brand A sequence are performed in intervals with non-sensitivemeasurements by the Brand C device. On the other hand, the measurementsequence of a Brand A device cannot overlap with that of a Brand Bdevice, because there is no contiguous range of non-sensitivemeasurements in the Brand B sequence large enough to accommodate therange of intervals in the Brand A sequence including sensitivemeasurements.

Returning to the method of FIG. 16C, configuration information such asthat of FIG. 17 may be used by the control circuit of an illuminationdevice in determining (for decision 1664) whether a measurement sequenceassociated with a detected measurement is known. In an embodiment forwhich the configuration information of FIG. 17 is used, a singledetection of an interfering measurement by another device would not initself be enough to determine whether which of the known measurementsequences is being used by the interfering device. If the interferingmeasurement sequence is not known, the control circuit initiates adetection process during the next interval to get further information (Nbranch of decision 1664 and step 1636). In the embodiment of FIG. 16C, achange of interval series after a maximum number of collisions isincluded (decision 1666 and 1668) to avoid an endless loop if thecontrol circuit is unable to determine the measurement sequence used bythe interfering device. This change to a different series of intervalsis similar to that described above for FIG. 16B.

In some embodiments, the control circuit is able to determine ameasurement sequence used by the interfering device by monitoring thecollision, free interval, and contiguous free interval counters duringsuccessive intervals. For example, a sequence of a detected photocurrentmeasurement (i.e., a collision), followed by a non-sensitive measurement(which increments the free interval and contiguous free intervalcounters), followed by another sensitive measurement (which incrementsthe collision counter and clears the contiguous free interval counter)indicates that the sequence of Brand A is used by the interferingdevice. A sequence of three sensitive measurements in a row, on theother hand, would indicate that the sequence of Brand C is used by theinterfering device.

If the sequence of the interfering measurements is known, the controlcircuit determines whether an overlapping, but non-interfering,measurement sequence by the controlled device is possible (decision1670). In an embodiment, configuration information such as that of FIG.17 is used to determine whether such an overlapping measurementconfiguration is possible. In addition to the considerations discussedabove in connection with FIG. 17, the control circuit may in anembodiment consider whether the measurement sequence of the controlleddevice should be changed. For example, in an embodiment for which aninterfering device uses a different measurement sequence than thecontrolled device, an overlapping measurement sequence may become easieror possible if the controlled device changes its measurement sequence tobe more compatible with that of the interfering device. Changing of adevice's measurement sequence may in some embodiments make prediction ofa device's behavior by other devices more difficult. However, inembodiments in which there are a limited number of measurement sequencesused and the illumination devices are capable of detecting the sequenceused by an interfering device, temporary adjustment of a device'smeasurement sequence may be a useful option for avoiding interference.

In the embodiment of FIG. 16C, if overlapping measurements are apossibility, the measurement sequence is revised if necessary to achievethe non-interfering overlap (decision 1670 and step 1672). Themeasurement sequence is started in the next interval if appropriate, ordelayed for a suitable number of intervals if needed to achieve anon-interfering measurement sequence (decision 1674 and step 1662). Ifoverlapping measurements are not possible, the control circuit changesto a different set of intervals and begins the detection sequence again(decision 1670, steps 1668 and 1634). In an alternate embodiment,another approach such as a delay time is used instead of changing to adifferent set of intervals. Variations of the method of FIG. 16C will berecognized by one of ordinary skill in the art in view of thisdisclosure. It is noted, for example, that configuration information forcompensation measurement sequences of illumination devices may be morecomplex than that shown in FIG. 17. Additional measurements may be takenin some embodiments, such as additional forward voltage measurementsusing alternate detectors. In some embodiments of illumination devicesstoring configuration information for other illumination devices,measurement sequences are not necessarily the same length for eachdevice. In embodiments for which non-sensitive measurements are notnecessarily non-interfering measurements, configuration information suchas that of FIG. 17 may include quantities defined separately forsensitive measurements and interfering measurements. Analysis in such anembodiment may be more complex than that described for FIG. 17.Variations of the methods of FIGS. 16A, 16B and 16C may be combined,resulting in many possible methods of avoiding interference-relatederror when performing compensation measurements for illuminationdevices.

Exemplary Embodiments of Improved Illumination Devices

The improved methods described herein for controlling an illuminationdevice may be used within substantially any LED illumination devicehaving a plurality of emission LED elements and one or morephotodetectors. As described in more detail below, the improved methodsdescribed herein may be implemented within an LED illumination device inthe form of hardware, software or a combination of both.

Illumination devices, which benefit from the improved methods describedherein, may have substantially any form factor including, but notlimited to, parabolic lamps (e.g., PAR 20, 30 or 38), linear lamps,flood lights and mini-reflectors. In some cases, the illuminationdevices may be installed in a ceiling or wall of a building, and may beconnected to an AC mains or some other AC power source. However, askilled artisan would understand how the improved methods describedherein may be used within other types of illumination devices powered byother power sources (e.g., batteries or solar energy).

Exemplary embodiments of an improved illumination device are describedwith reference to FIGS. 18-21, which show different types of LEDillumination devices, each having one or more emitter modules. Althoughexamples are provided herein, the present invention is not limited toany particular type of LED illumination device or emitter module design.A skilled artisan would understand how the method steps described hereinmay be applied to other types of LED illumination devices havingsubstantially different emitter module designs.

FIG. 18A is a photograph of a linear lamp 1810 comprising a plurality ofemitter modules (not shown in FIG. 18A), which are spaced apart from oneanother and arranged generally in a line. In an embodiment, each emittermodule included within linear lamp 1810 includes a plurality of emissionLEDs and at least one dedicated photodetector, all of which are mountedonto a common substrate and encapsulated within a primary opticsstructure. The primary optics structure may be formed from a variety ofdifferent materials and may have substantially any shape and/ordimensions necessary to shape the light emitted by the emission LEDs ina desirable manner. Although the primary optics structure is describedbelow as a dome, one skilled in the art would understand how the primaryoptics structure may have substantially any other shape orconfiguration, which encapsulates the emission LEDs and the at least onephotodetector.

A computer-generated representation of a top view of an exemplaryemitter module 1820 that may be included within the linear lamp 1810 ofFIG. 18A is shown in FIG. 18B. In the illustrated embodiment, emittermodule 1820 includes four differently colored emission LEDs 1830, whichare arranged in a square array and placed as close as possible togetherin the center of a primary optics structure (e.g., a dome) 1840, so asto approximate a centrally located point source. In some embodiments,the emission LEDs 1830 may each be configured for producing illuminationat a different peak emission wavelength. For example, the emission LEDs1830 may include RGBW LEDs or RGBY LEDs. In addition to the emissionLEDs 1830, a dedicated photodetector 1850 is included within the dome1840 and arranged somewhere around the periphery of the emission LEDarray. The dedicated photodetector 1850 may be any device (such as asilicon photodiode or an LED) that produces current indicative ofincident light.

FIGS. 19A and 19B illustrate a substantially different type ofillumination device and emitter module design. Specifically, FIG. 19Adepicts an illumination device 1910 having a parabolic form factor(e.g., a PAR 38) and a single emitter module (not shown in FIG. 19A). Asthese illumination devices have only one emitter module, the emittermodules included in such devices typically include a plurality ofdifferently colored chains of LEDs (LED elements), where each chainincludes two or more LEDs of the same color. FIG. 19B illustrates anexemplary emitter module 1920 that may be included within the PAR lamp1910 shown in FIG. 19A.

In the illustrated embodiment, emitter module 1920 includes an array ofemission LEDs 1930 and a plurality of dedicated photodetectors 1950, allof which are mounted on a common substrate and encapsulated within aprimary optics structure (e.g., a dome) 1940. In some embodiments, thearray of emission LEDs 1930 may include a number of differently coloredchains of LEDS, wherein each chain is configured for producingillumination at a different peak emission wavelength. According to oneembodiment, the array of emission LEDs 1930 may include a chain of fourred LEDs, a chain of four green LEDs, a chain of four blue LEDs, and achain of four white or yellow LEDs. Each chain of LEDs is coupled inseries and driven with the same drive current. In some embodiments, theindividual LEDs in each chain may be scattered about the array, andarranged so that no color appears twice in any row, column or diagonal,to improve color mixing within the emitter module 1920.

In the exemplary embodiment of FIG. 19B, four dedicated photodetectors1950 are included within the dome 1940 and arranged around the peripheryof the array. In some embodiments, the dedicated photodetectors 1950 maybe placed close to, and in the middle of, each edge of the array and maybe connected in parallel to a receiver of the illumination device. Byconnecting the dedicated photodetectors 1950 in parallel with thereceiver, the photocurrents induced on each photodetector may be summedto minimize the spatial variation between the similarly colored LEDs,which may be scattered about the array. The dedicated photodetectors1950 may be any devices that produce current indicative of incidentlight (such as a silicon photodiode or an LED). In one embodiment,however, the dedicated photodetectors 1950 are preferably LEDs with peakemission wavelengths in the range of 500 nm to 700 nm. Photodetectorswith such peak emission wavelengths will not produce photocurrent inresponse to infrared light, which reduces interference from ambientlight. To the extent some amount of ambient light is nonethelessdetectable during, for example, a photocurrent measurement, methods asdescribed herein may be used to minimize compensation errors caused bysuch ambient light. For example, effects of a constant ambientillumination on a photocurrent measurement may be removed by subtractionas discussed above. In the case of non-constant external illumination,methods as described herein may be used to avoid taking photocurrentmeasurements in the presence of such non-constant illumination.

The illumination devices shown in FIGS. 18A and 19A and the emittermodules shown in FIGS. 18B and 19B are provided merely as examples ofillumination devices in which the interference-resistant compensationmethods described herein may be used. Further description of theseillumination devices and emitter modules may be found in U.S. patentapplication Ser. No. 14/097,339 and U.S. Provisional Patent ApplicationNo. 61/886,471, which are commonly assigned and incorporated herein byreference in their entirety. Still further description of additionalemitter module embodiments may be found in co-pending U.S. patentapplication Ser. No. 14/314,530. However, the inventive conceptsdescribed herein are not limited to any particular type of LEDillumination device, any particular number of emitter modules that maybe included within an LED illumination device, or any particular number,color or arrangement of emission LEDs and photodetectors that may beincluded within an emitter module. Instead, the methods described hereinmay contemplate only an LED illumination device including a plurality ofemission LEDs and at least one photodetector. In some embodiments, adedicated photodetector may not be required, if one or more of theemission LEDs is configured, at times, to provide such functionality.

FIG. 20 is one example of a block diagram of an illumination device 2000configured to avoid interference-related errors when compensating forvariations in parameters such as drive current, temperature, and LEDcharacteristics. The illumination device illustrated in FIG. 20 providesone example of the hardware and/or software that may be used toimplement interference-resistant measurement methods such as those shownin FIGS. 16A through 16C.

In the illustrated embodiment, illumination device 2000 comprises aplurality of emission LED elements 2045 and one or more dedicatedphotodetectors 2050. The emission LED elements 2045, in this example,comprise four chains of any number of LEDs. In typical embodiments, eachchain may have 2 to 4 LEDs of the same color, which are coupled inseries and configured to receive the same drive current. In one example,the emission LED elements 2045 may include a chain of red LEDs, a chainof green LEDs, a chain of blue LEDs, and a chain of white or yellowLEDs. However, the methods and devices described herein are not limitedto any particular number of LED chains, any particular number of LEDswithin the chains, or any particular color or combination of LED colors.

Similarly, the methods and devices described herein are not limited toany particular type, number, color, combination or arrangement ofphotodetectors. In one embodiment, the one or more dedicatedphotodetectors 2050 may include a small red, orange or yellow LED. Inanother embodiment, the one or more dedicated photodetectors 128 mayinclude one or more small red LEDs and one or more small green LEDs. Insome embodiments, one or more of the dedicated photodetector(s) 2050shown in FIG. 20 may be omitted if one or more of the emission LEDs 2045is configured, at times, to function as a photodetector. The pluralityof emission LEDs 2045 and the (optional) dedicated photodetectors 2050may be included within an emitter module, as discussed above. In someembodiments, an illumination device may include more than one emittermodule, as discussed above.

In addition to including one or more emitter modules, illuminationdevice 2000 includes various hardware and software components, which areconfigured for powering the illumination device and controlling thelight output from the emitter module(s). In one embodiment, theillumination device is connected to AC mains 2005, and includes an AC/DCconverter 2010 for converting AC mains power (e.g., 120V or 240V) to aDC voltage (V_(DC)). As shown in FIG. 20, this DC voltage (e.g., 15V) issupplied to the LED driver and receiver circuit 2040 for producing theoperative drive currents applied to the emission LEDs 2045 for producingillumination. In addition to the AC/DC converter, a DC/DC converter 2015is included for converting the DC voltage V_(DC) (e.g., 15V) to a lowervoltage V_(L) (e.g., 3.3V), which is used to power the low voltagecircuitry included within the illumination device, such as PLL 2020,wireless interface 2025, and control circuit 2035.

In the illustrated embodiment, PLL 2020 locks to the AC mains frequency(e.g., 50 or 60 HZ) and produces a high speed clock (CLK) signal and asynchronization signal (SYNC). The CLK signal provides the timing forcontrol circuit 2035 and LED driver and receiver circuit 2040. In oneexample, the CLK signal frequency is in the tens of MHz range (e.g., 23MHz), and is precisely synchronized to the AC Mains frequency and phase.The SYNC signal is used by the control circuit 2035 to create the timingof the intervals used for the detection and compensation measurementsdescribed above. In one example, the SYNC signal frequency is equal tothe AC Mains frequency (e.g., 50 or 60 HZ) and also has a precise phasealignment with the AC Mains. In another embodiment, the SYNC signalfrequency is an integral multiple of the AC mains frequency. In anembodiment, timing reference signal 1520 of FIG. 15 is an example of theSYNC signal of FIG. 20.

In some embodiments, a wireless interface 2025 may be included and usedto calibrate the illumination device 2000 during manufacturing. Asdiscussed in the co-pending applications referenced herein, an externalcalibration tool (not shown in FIG. 20) may communicate calibrationvalues (e.g., luminous flux, chromaticity and/or other opticalmeasurement values) to an illumination device under test via thewireless interface 2025. The calibration values received via thewireless interface 2025 may be stored in the table of calibration valueswithin a storage medium 2030 of the control circuit 2035, for example.In some embodiments, the control circuit 2035 may use the calibrationvalues to generate calibration coefficients, which are stored within thestorage medium 2030 in addition to, or in lieu of, the receivedcalibration values.

Wireless interface 2025 is not limited to receiving only calibrationdata, and may be used for communicating information and commands formany other purposes. For example, wireless interface 2025 could be usedduring normal operation to communicate commands, which may be used tocontrol the illumination device 2000, or to obtain information about theillumination device 2000. For instance, commands may be communicated tothe illumination device 2000 via the wireless interface 2025 to turn theillumination device on/off, to control the dimming level and/or colorset point of the illumination device, to initiate the calibrationprocedure, or to store calibration results in memory. In other examples,wireless interface 2025 may be used to obtain status information orfault condition codes associated with illumination device 2000.

In some embodiments, wireless interface 2025 could operate according toZigBee, WiFi, Bluetooth, or any other proprietary or standard wirelessdata communication protocol. In other embodiments, wireless interface2025 could communicate using radio frequency (RF), infrared (IR) lightor visible light. In alternative embodiments, a wired interface could beused, in place of the wireless interface 2025 shown, to communicateinformation, data and/or commands over the AC mains or a dedicatedconductor or set of conductors.

Using the timing signals received from PLL 2020, the control circuit2035 calculates and produces values indicating the desired drive currentto be used for each LED chain 2045. This information may be communicatedfrom the control circuit 2035 to the LED driver and receiver circuit2040 over a serial bus conforming to a standard, such as SPI or I²C, forexample. In addition, the control circuit 2035 may provide a latchingsignal that instructs the LED driver and receiver circuit 2040 tosimultaneously change the drive currents supplied to each of the LEDs2045 to prevent brightness and color artifacts.

Control circuit 2035 may be configured for determining the respectivedrive currents needed to achieve a desired luminous flux and/or adesired chromaticity for the illumination device in accordance with oneor more compensation methods as described above in connection with FIGS.6-9 and described further in the co-pending applications referencedherein. Control circuit 2035 is further configured for operationsdescribed herein in connection with avoiding interference. Depending onthe particular embodiment such operations include, for example,determining whether an interfering photocurrent measurement is made byanother device during a detection interval or measurement interval,waiting for a delay time before continuing to monitor detectionintervals, changing to a different series of intervals, determiningwhether detection has indicated that compensation measurements may bestarted without likely interference, or determining the measurementsequence used by an interfering device.

In some embodiments, the control circuit 2035 may determine therespective drive currents and perform the interference-relatedoperations described herein by executing program instructions storedwithin the storage medium 2030. In one embodiment, the storage mediummay be a non-volatile memory, and may be configured for storing theprogram instructions along with a table of calibration values used inthe compensation methods and a data structure including configurationinformation such as that of FIG. 17. Alternatively, the control circuit2035 may include combinational logic for determining the desired drivecurrents or performing other operations, such that program instructionsfor determining drive currents are not stored on storage medium 2030. Ina further embodiment, operations of control circuit 2035 may be carriedout using a combination of program instructions and combinational logic.Storage medium 2030, along with other memory or storage describedherein, includes a plurality of storage locations addressable by controlcircuit 2035 or a processor such as that associated with controller 2190in FIG. 21 for storing software programs and data associated with themethods described herein. As such, storage medium 2030 and other memoryor storage media described herein may be implemented using anycombination of built-in volatile or non-volatile memory, includingrandom-access memory (RAM) and read-only memory (ROM) and integrated orperipheral storage devices such as magnetic disks, optical disks, solidstate drives or flash drives. In an embodiment, storage medium 2030 maybe used to store one or more counters such as the collision counter,free interval counter, and contiguous free interval counters describedin connection with FIGS. 16B and 16C above.

In general, the LED driver and receiver circuit 2040 may include anumber (N) of driver blocks 2115 equal to the number of emission LEDchains 2045 included within the illumination device. In the exemplaryembodiment discussed herein, LED driver and receiver circuit 2040comprises four driver blocks 2115, each configured to produceillumination from a different one of the emission LED chains 2045. TheLED driver and receiver circuit 2040 also comprises the circuitry neededto measure ambient temperature (optional), the detector and/or emitterforward voltages, and the detector photocurrents, and to adjust the LEDdrive currents accordingly. Each driver block 2115 receives dataindicating a desired drive current from the control circuit 2035, alongwith a latching signal indicating when the driver block 2115 shouldchange the drive current.

FIG. 21 is an exemplary block diagram of an LED driver and receivercircuit 2040, according to one embodiment of the invention. As shown inFIG. 21, the LED driver and receiver circuit 2040 includes four driverblocks 2115, each block including a buck converter 2120, a currentsource 2125, and an LC filter 2145 for generating the drive currentsthat are supplied to a connected emission LED element 2045(a) to produceillumination and obtain forward voltage (Vfe) measurements. In someembodiments, buck converter 2120 may produce a pulse width modulated(PWM) voltage output (Vdr) when the controller 2190 drives the “Out_En”signal high. This voltage signal (Vdr) is filtered by the LC filter 2145to produce a forward voltage on the anode of the connected LED chain2045(a). The cathode of the LED chain is connected to the current source2125, which forces a fixed drive current equal to the value provided bythe “Emitter Current” signal through the LED chain 2045(a) when the“Led_On” signal is high. The “Vc” signal from the current source 2125provides feedback to the buck converter 2120 to output the proper dutycycle and minimize the voltage drop across the current source 2125.

As shown in FIG. 21, each driver block 2115 includes a differenceamplifier 2140 for measuring the forward voltage drop (Vfe) across thechain of emission LEDs 2045a. When measuring Vfe, the buck converter2120 is turned off and the current source 2125 is configured for drawinga relatively small drive current (e.g., about 1 mA) through theconnected chain of emission LEDs 2045(a). The voltage drop (Vfe)produced across the LED chain 2045(a) by that current is measured by thedifference amplifier 2140. The difference amplifier 2140 produces asignal that is equal to the forward voltage (Vfe) drop across theemission LED chain 2045(a) during forward voltage measurements.

As noted above, some embodiments of the invention may use one of theemission LEDs (e.g., a green emission LED), at times, as aphotodetector. In such embodiments, the driver blocks 2115 may includeadditional circuitry for measuring the photocurrents (Iph_d2), which areinduced across an emission LED, when the emission LED is configured fordetecting incident light. For example, each driver block 2115 mayinclude a transimpedance amplifier 2130, which generally functions toconvert an input current to an output voltage proportional to a feedbackresistance. As shown in FIG. 21, the positive terminal of transimpedanceamplifier 2130 is connected to the Vdr output of the buck converter2120, while the negative terminal is connected to the cathode of thelast LED in the LED chain 2045(a). Transimpedance amplifier 2130 isenabled when the “LED_On” signal is low. When the “LED_On” signal ishigh, the output of transimpedance amplifier 2130 is tri-stated.

When measuring the photocurrents (Iph_d2) induced by an emission LED,the buck converters 2120 connected to all other emission LEDs should beturned off to avoid visual artifacts produced by LED current transients.In addition, the buck converter 2120 coupled to the emission LED undertest should also be turned off to prevent switching noise within thebuck converter from interfering with the photocurrent measurements.Although turned off, the Vdr output of the buck converter 2120 coupledto the emission LED under test is held to a particular value (e.g.,about 2-3.5 volts times the number of emission LEDs in the chain) by thecapacitor within LC filter 2145. When this voltage (Vdr) is supplied tothe anode of emission LED under test and the positive terminal of thetransimpedance amplifier 2130, the transimpedance amplifier produces anoutput voltage (relative to Vdr) that is supplied to the positiveterminal of difference amplifier 2135. Difference amplifier 2135compares the output voltage of transimpedance amplifier 2130 to Vdr andgenerates a difference signal, which corresponds to the photocurrent(Iph_d2) induced across the LED chain 2045(a).

In addition to including a plurality of driver blocks 2115, the LEDdriver and receiver circuit 2040 may include one or more receiver blocks2150 for measuring the forward voltages (Vfd) and photocurrents (Iph_d1or Iph_d2) induced across the one or more dedicated photodetectors 2050.Although only one receiver block 2150 is shown in FIG. 21, the LEDdriver and receiver circuit 2040 may generally include a number ofreceiver blocks 2150 equal to the number of dedicated photodetectorsincluded within the emitter module.

In the illustrated embodiment, receiver block 2150 comprises a voltagesource 2155, which is coupled for supplying a DC voltage (Vdr) to theanode of the dedicated photodetector 2050 coupled to the receiver block,while the cathode of the photodetector 2050 is connected to currentsource 2160. When photodetector 2050 is configured for obtaining forwardvoltage (Vfd), the controller 2190 supplies a “Detector_On” signal tothe current source 2160, which forces a fixed drive current (Idrv) equalto the value provided by the “Detector Current” signal throughphotodetector 2050.

When obtaining detector forward voltage (Vfd) measurements, currentsource 2160 is configured for drawing a relatively small amount of drivecurrent (Idrv) through photodetector 2050. The voltage drop (Vfd)produced across photodetector 2050 by that current is measured bydifference amplifier 2175, which produces a signal equal to the forwardvoltage (Vfd) drop across photodetector 2050. As noted above, the drivecurrent (Idrv) forced through photodetector 2050 by the current source2160 is generally a relatively small, non-operative drive current. Inthe embodiment in which four dedicated photodetectors 2050 are coupledin parallel, the non-operative drive current may be roughly 1 mA.However, smaller/larger drive currents may be used in embodiments thatinclude fewer/greater numbers of photodetectors, or embodiments that donot connect the photodetectors in parallel.

Similar to driver block 2115, receiver block 2150 also includescircuitry for measuring the photocurrents (Iph_d1 or Iph_d2) induced onphotodetector 2050 by ambient light, as well as light emitted by theemission LEDs. As shown in FIG. 21, the positive terminal oftransimpedance amplifier 2165 is coupled to the Vdr output of voltagesource 2155, while the negative terminal is connected to the cathode ofphotodetector 2050. When connected in this manner, the transimpedanceamplifier 2165 produces an output voltage relative to Vdr (e.g., about0-1V), which is supplied to the positive terminal of differenceamplifier 2170. Difference amplifier 2170 compares the output voltage toVdr and generates a difference signal, which corresponds to thephotocurrent (Iph_d1 or Iph_d2) induced across photodetector 2050.Transimpedance amplifier 2165 is enabled when the “Detector_On” signalis low. When the “Detector_On” signal is high, the output oftransimpedance amplifier 2165 is tri-stated.

As noted above, some embodiments of the invention may scatter theindividual LEDs within each chain of LEDs 2045 about the array of LEDs,so that no two LEDs of the same color exist in any row, column ordiagonal (see, e.g., FIG. 19B). By connecting a plurality of dedicatedphotodetectors 2050 in parallel with the receiver block 2150, thephotocurrents (Iph_d1 or Iph_d2) induced on each photodetector 2050 bythe LEDs of a given color may be summed to minimize the spatialvariation between the similarly colored LEDs, which are scattered aboutthe array.

As shown in FIG. 21, the LED driver and receiver circuit 2040 may alsoinclude a multiplexor (Mux) 2180, an analog to digital converter (ADC)2185, a controller 2190, and an optional temperature sensor 2195. Insome embodiments, multiplexor 2180 may be coupled for receiving theemitter forward voltage (Vfe) and the (optional) photocurrent (Iph_d2)measurements from the driver blocks 2115, and the detector forwardvoltage (Vfd) and detector photocurrent (Iph_d1 and/or Iph_d2)measurements from the receiver block 2150. The ADC 2185 digitizes theemitter forward voltage (Vfe) and the optional photocurrent (Iph_d2)measurements output from the driver blocks 2115, and the detectorforward voltage (Vfd) and detector photocurrent (Iph_d1 and/or Iph_d2)measurements output from the receiver block 2150, and provides theresults to the controller 2190. The controller 2190 determines when totake forward voltage and photocurrent measurements and produces theOut_En, Emitter Current and Led_On signals, which are supplied to thedriver blocks 2115, and the Detector Current and Detector_On signals,which are supplied to the receiver block 2150 as shown in FIG. 21.

In some embodiments, the LED driver and receiver circuit 2040 mayinclude an optional temperature sensor 2195 for taking ambienttemperature (Ta) measurements. In such embodiments, multiplexor 2180 mayalso be coupled for multiplexing the ambient temperature (Ta) with theforward voltage and photocurrent measurements sent to the ADC 2185. Insome embodiments, the temperature sensor 2195 may be a thermistor, andmay be included on the driver circuit chip for measuring the ambienttemperature surrounding the LEDs, or a temperature from the heat sink ofthe emitter module. If the optional temperature sensor 2195 is included,the output of the temperature sensor may be used in some embodiments todetermine if a significant change in temperature is detected. In someembodiments detection of a significant change in temperature may causecompensation measurements to be initiated.

One implementation of an improved illumination device 2000 has now beendescribed in reference to FIGS. 20-21. Further description of such anillumination device may be found in commonly assigned U.S. applicationSer. Nos. 13/970,944; 13/970,964; 13/970,990; and 14/097,339. A skilledartisan would understand how the illumination device could bealternatively implemented within the scope of the methods and devicesdescribed herein.

An exemplary block diagram of circuit components for an illuminationdevice including multiple emitter modules is shown in FIG. 22. In theembodiment of FIG. 22, the circuit components are housed on a powersupply board 2202 and emitter board 2204 which are dimensioned to fitwithin the housing of a linear illumination device. An external view ofan embodiment of such a linear illumination device is shown in FIG. 18A.Emitter board 2204 in the embodiment of FIG. 22 includes 6 emittermodules 2212 arranged in a linear row. A representation of a top view ofan exemplary embodiment of emitter module 2212 is shown in FIG. 18B.

In the embodiment of FIG. 22, power supply board 2202 comprises AC/DCconverter 2206 and controller 2208. AC/DC converter 2206 converters ACmains power to a DC voltage of typically 15-20V, which is then used topower controller 2208 and emitter board 2204. The DC voltage from AC/DCconverter 2206 may be converted to lower voltages as well elsewherewithin the illumination device. Controller 2208 communicates withemitter board 2204 through a digital control bus, in this example.Controller 2208 could comprise a wireless, power line, or any other typeof communication interface to enable the color of the linearillumination device to be adjusted. In an embodiment, controller 2208also provides to each of interface circuits 2210 a timing signal and anoffset from the timing signal at which measurement intervals and/ordetection intervals for the associated emitter module are to occur. In afurther embodiment, adjacently positioned emitter modules within theillumination device are assigned different offsets from the timingreference, so that compensation measurements performed by adjacentemitter modules are performed using non-overlapping sets of intervals.In one such embodiment, an illumination device including six emittermodules such as that illustrated in FIG. 22 uses three different offsetsfrom a timing reference: a first offset for the first and fourth emittermodules (counting from one end of the device), a second offset for thesecond and fifth emitter modules, and a third offset for the third andsixth emitter modules. In alternative embodiments a different number ofoffsets may be used, including the use of a different offset for eachindividual emitter module.

In the illustrated embodiment, emitter board 2204 comprises six emittermodules 2212 and six interface circuits 2210. Interface circuits 2210communicate with controller 2208 over the digital control bus andproduce the drive currents supplied to the LEDs within the emittermodules 2212. FIG. 23 illustrates exemplary circuitry that may beincluded within interface circuitry 2210 and emitter modules 2212.Interface circuitry 2210 comprises control logic 2302, LED drivers 2304,and receiver 2306. Emitter module 2212 comprises emission LEDs 2308 anda detector 2310. Control logic 2302 may comprise a microcontroller orspecial logic, and communicates with controller 2208 over the digitalcontrol bus. Control logic 2302 also sets the drive current produced byLED drivers 2304 to adjust the color and/or intensity of the lightproduced by emission LEDs 2308, and manages receiver 2306 to monitor thelight produced by each individual LED 2308 via detector 2310. In someembodiments, control logic 2302 may comprise memory for storingcalibration information necessary for maintaining precise color, oralternatively, such information could be stored in controller 2208.Similarly, other information used in performing the methods describedherein is in some embodiments stored in memory locations within controllogic 2302, within controller 2208, or distributed between both of thesecircuits. Such other information may include configuration informationsuch as that discussed in connection with FIG. 17 above.

In an embodiment, the circuit components on power supply board 2202 areimplemented in a similar manner as the power supply and controlcircuitry shown in FIG. 20, including AC/DC converter 2010, DC/DCconverter 2015, PLL 2020, wireless interface 2025, and control circuit2035. Similarly, interface circuit 2210 is in some embodimentsimplemented in a manner similar to driver and receiver circuit 2040shown in FIGS. 20-21. LEDs 2308 and detector 2310 are in someembodiments implemented using LED chains 2045 and detectors 2050 of FIG.20, respectively. Functions of control circuit 2035 in FIG. 20 may insome embodiments be distributed between control logic 2302 of FIG. 23and controller 2208 of FIG. 22. In some embodiments, certain functionsof control circuit 2035 may be duplicated in both controller 2208 andcontrol logic 2302. Controller 2208 may also be referred to as a devicecontrol circuit herein. In an embodiment, the device control circuit isconfigured to control the entire illumination device. Control logic 2302may also be referred to herein as a module control circuit for itsrespective emitter module 2212. In an embodiment, the module controlcircuit is configured to control functionality of its respective emittermodule, including performance of compensation measurements andadjustment of illumination settings. Certain functions of the modulecontrol circuits may in some embodiments be performed by the devicecontrol circuit 2208.

One implementation of an improved illumination device has now beendescribed in reference to FIGS. 22-23. Further description of such anillumination device may be found in commonly assigned U.S. applicationSer. Nos. 13/970,944; 13/970,964; 13/970,990; and 14/097,339. A skilledartisan would understand how the illumination device could bealternatively implemented within the scope of the methods and devicesdescribed herein.

It will be appreciated to those skilled in the art having the benefit ofthis disclosure that this invention is believed to provide an improvedillumination device and methods for avoiding interference-related errorswhen compensating individual LEDs in the illumination device forvariations in quantities such as drive current and temperature. Furthermodifications and alternative embodiments of various aspects of theinvention will be apparent to those skilled in the art in view of thisdescription. It is intended, therefore, that the following claims beinterpreted to embrace all such modifications and changes and,accordingly, the specification and drawings are to be regarded in anillustrative rather than a restrictive sense.

What is claimed is:
 1. A method for controlling an illumination devicecomprising multiple emitter modules, wherein each emitter modulecomprises multiple emission light emitting diode (LED) elements and oneor more photodetectors, the method comprising: operating one or more ofthe multiple emission LED elements in each of the multiple emittermodules to produce illumination substantially continuously by supplyinga respective drive current at an operative drive current level to eachof the one or more of the multiple emission LED elements; bringing therespective drive currents of all except one of the emission LED elementswithin a first emitter module of the multiple emitter modules to anon-operative drive current level, which is insufficient to produceillumination, for the duration of a measurement interval within a firstseries of measurement intervals interspersed with periods of saidillumination; and bringing the respective drive currents of all exceptone of the emission LED elements within a second emitter module of themultiple emitter modules to a non-operative drive current level, whichis insufficient to produce illumination, for the duration of ameasurement interval within a second series of measurement intervalsinterspersed with periods of said illumination, wherein the first seriesof measurement intervals and the second series of measurement intervalsare separated by a respective first offset and second offset from atiming reference.
 2. The method of claim 1, for either of the first andsecond emitter modules, further comprising: during the measurementinterval within the respective first or second series of measurementintervals, applying an operative drive current level, which issufficient to produce illumination, to the one of the emission LEDelements; and during said applying an operative drive current level tothe one of the emission LED elements, monitoring a respective first orsecond measurement photocurrent induced in the one or morephotodetectors included within the emitter module.
 3. The method ofclaim 2, for either of the first or second emitter modules, furthercomprising bringing the drive current applied to the one of the emissionLED elements to a non-operative drive current level, which isinsufficient to produce illumination, for a portion of the respectivemeasurement interval, such that the respective drive currents of all ofthe emission LED elements within the respective emitter module are at anon-operative drive current level for the portion of the respectivemeasurement interval.
 4. The method of claim 3, for either of the firstor second emitter modules and during the portion of the respectivemeasurement interval, further comprising monitoring a respective firstor second background photocurrent induced in the one or morephotodetectors included within the emitter module.
 5. The method ofclaim 4, for either of the first or second emitter modules, furthercomprising subtracting the respective first or second backgroundphotocurrent from the respective first or second measurementphotocurrent.
 6. The method of claim 5, for either of the first orsecond emitter modules, further comprising storing a result of saidsubtracting as a respective first or second corrected photocurrent. 7.The method of claim 6, wherein said storing a result of said subtractingis in response to a determination that the result is within an expectedrange.
 8. The method of claim 1, wherein the timing reference comprisesa periodic timing signal.
 9. The method of claim 8, wherein the timingreference is derived from an AC mains signal.
 10. The method of claim 1,wherein the multiple emitter modules consist of one or more sets ofthree emitter modules, and wherein each emitter module within a set usesa respective series of measurement intervals having a different offsetfrom the timing reference than that used by the other emitter moduleswithin the set.
 11. An illumination device comprising: multiple emittermodules, wherein each emitter module comprises multiple emission lightemitting diode (LED) elements and one or more photodetectors; and acontrol circuit operably coupled to the multiple emitter modules,wherein the control circuit is adapted to: operate one or more of themultiple emission LED elements within each of the multiple emittermodules to produce illumination substantially continuously by supplyinga respective drive current at an operative drive current level to eachof the one or more of the multiple emission LED elements; bring therespective drive currents of all except one of the emission LED elementswithin a first emitter module of the multiple emitter modules to anon-operative drive current level, which is insufficient to produceillumination, for the duration of a measurement interval within a firstseries of measurement intervals interspersed with periods of saidillumination; and bring the respective drive currents of all except oneof the emission LED elements within a second emitter module of themultiple emitter modules to a non-operative drive current level, whichis insufficient to produce illumination, for the duration of ameasurement interval within a second series of measurement intervalsinterspersed with periods of said illumination, wherein the first seriesof measurement intervals and the second series of measurement intervalsare separated by a respective first offset and second offset from atiming reference.
 12. The illumination device of claim 11, furthercomprising a timing reference generator operatively coupled to thecontrol circuit and adapted to generate the timing reference.
 13. Theillumination device of claim 12, wherein the timing reference comprisesa periodic timing signal and the timing reference generator comprises aphase-locked loop.
 14. The illumination device of claim 11, furthercomprising multiple driver circuits operably coupled to respectiveemitter modules of the multiple emitter modules and to the controlcircuit, and wherein the control circuit is configured to adjust a drivecurrent of an LED element within an emitter module by providing a drivecurrent setting to a respective driver circuit for the emitter module.15. The illumination device of claim 11, wherein, for each of the firstand second emitter modules, the control circuit is further adapted to:during the measurement interval within the respective first or secondseries of measurement intervals, apply an operative drive current level,which is sufficient to produce illumination, to the one of the emissionLED elements; and during said applying the operative drive current levelto the one of the emission LED elements, monitor a respective first orsecond measurement photocurrent induced in the one or morephotodetectors included within the emitter module.
 16. The illuminationdevice of claim 15, wherein, for each of the first and second emittermodules, the control circuit is further adapted to: bring the drivecurrent applied to the one of the emission LED elements to anon-operative drive current level, which is insufficient to produceillumination, for a portion of the respective measurement interval, suchthat the respective drive currents of all of the emission LED elementswithin the respective emitter module are at a non-operative drivecurrent level for the portion of the respective measurement interval;and during the portion of the respective measurement interval, monitor arespective first or second background photocurrent induced in the one ormore photodetectors included within the emitter module.
 17. Theillumination device of claim 16, wherein, for each of the first andsecond emitter modules, the control circuit is further adapted tosubtract the respective first or second background photocurrent from therespective first or second measurement photocurrent.
 18. Theillumination device of claim 17, further comprising a plurality ofstorage locations accessible by the control circuit, and wherein thecontrol circuit is further adapted to store a result of subtracting thefirst or second background photocurrent from the first or secondmeasurement photocurrent in one or more of the storage locations as afirst or second corrected photocurrent.
 19. The illumination device ofclaim 18, wherein the control circuit is further adapted to determinewhether the result is within an expected range and store the result inresponse to a determination that the result is within an expected range.20. The illumination device of claim 11, wherein the multiple emittermodules consist of one or more sets of three emitter modules, andwherein the control circuit is further adapted to use, for each emittermodule within a set, a respective measurement interval having adifferent offset from the timing reference than that of the otheremitter modules within the set.
 21. The illumination device of claim 11,wherein the control circuit comprises a respective module controlcircuit for each emitter module within the illumination device.
 22. Theillumination device of claim 21, wherein the control circuit furthercomprises a device control circuit adapted to provide to each of themodule control circuits a respective offset from the timing referencefor the respective series of measurement intervals used by therespective emitter module.
 23. A method for controlling an illuminationdevice comprising first and second light emitting diode (LED) elements,the method comprising: operating one or more of the first and second LEDelements to produce illumination by supplying a respective drive currentat an operative drive current level to each of the one or more of thefirst and second LED elements; bringing the respective drive currents ofall except the first LED element to a non-operative drive current levelfor a duration of a first measurement interval within a first series ofmeasurement intervals; and bringing the respective drive currents of allexcept the second LED element to a non-operative drive current level fora duration of a second measurement interval within a second series ofmeasurement intervals, wherein the first series of measurement intervalsand the second series of measurement intervals are separated by arespective first offset and second offset from a timing reference. 24.The method of claim 23, for either of the first or second LED elements,further comprising: during the measurement interval within therespective first or second series of measurement intervals, applying anoperative drive current level, which is sufficient to produceillumination, to the respective one of the LED elements; and during theapplying the operative drive current level to the respective one of theLED elements, monitoring a respective first or second measurementphotocurrent induced in at least one photodetector.
 25. The method ofclaim 24, for either of the first or second LED elements, furthercomprising: bringing the drive current applied to the respective one ofthe LED elements to a non-operative drive current level for a portion ofthe respective first or second measurement interval, such that therespective drive currents of all of the LED elements are at anon-operative drive current level for the portion of the respectivemeasurement interval; and during the portion of the respectivemeasurement interval, monitoring a respective first or second backgroundphotocurrent induced in the at least one photodetector.
 26. The methodof claim 25, for either of the first or second LED elements, furthercomprising: subtracting the respective first or second backgroundphotocurrent from the respective first or second measurementphotocurrent; and storing a respective result of the subtracting inresponse to a determination that the respective result is within anexpected range.
 27. The method of claim 23, wherein the timing referenceis derived from an AC mains signal.
 28. An illumination devicecomprising: first and second light emitting diode (LED) elements; and acontrol circuit operably coupled to the first and the second LEDelements, wherein the control circuit is adapted to: operate the firstand second LED elements to produce illumination by supplying arespective drive current at an operative drive current level to each ofthe first and second LED elements; bring the respective drive currentsof all except the first LED element to a non-operative drive currentlevel for a duration of a first measurement interval within a firstseries of measurement intervals; and bring the respective drive currentsof all except the second LED element to a non-operative drive currentlevel for a duration of a second measurement interval within a secondseries of measurement intervals, wherein the first series of measurementintervals and the second series of measurement intervals are separatedby a respective first offset and second offset from a timing reference.29. The illumination device of claim 28, further comprising a timingreference generator operatively coupled to the control circuit andadapted to generate the timing reference from an AC mains signal. 30.The illumination device of claim 28, wherein, for each of the first andsecond LED elements, the control circuit is further adapted to: duringthe measurement interval within the respective first or second series ofmeasurement intervals, apply an operative drive current level, which issufficient to produce illumination, to the respective one of the LEDelements; and during the applying the operative drive current level tothe respective one of the LED elements, monitor a respective first orsecond measurement photocurrent induced in at least one photodetector.31. The illumination device of claim 30, wherein, for each of the firstand second LED elements, the control circuit is further adapted to:bring the drive current applied to the one of the LED elements to anon-operative drive current level for a portion of the respectivemeasurement interval, such that the respective drive currents of all ofthe LED elements are at a non-operative drive current level for theportion of the respective measurement interval; and during the portionof the respective measurement interval, monitor a respective first orsecond background photocurrent induced in the at least onephotodetector.
 32. The illumination device of claim 31, wherein, foreach of the first and second LED elements, the control circuit isfurther adapted to: subtract the respective first or second backgroundphotocurrent from the respective first or second measurementphotocurrent; and store in one or more storage locations a respectiveresult of the subtracting in response to a determination that therespective result is within an expected range.
 33. A method forcontrolling an illumination device comprising first and second lightemitting diode (LED) elements, the method comprising: supplying arespective drive current at an operative drive current level to each ofthe first LED element and the second LED element to produceillumination; during a measurement interval: measuring a firstphotocurrent induced in at least one photodetector while the respectivedrive current of the first LED element is supplied at an operative drivecurrent level to produce illumination and the respective drive currentof the second LED element is supplied at a non-operative drive currentlevel; and measuring a second photocurrent induced in the at least onephotodetector while the respective drive current of each of the firstLED element and the second LED element is supplied at a non-operativedrive current level; subtracting the second photocurrent from the firstphotocurrent to obtain a corrected photocurrent; and at an end of themeasurement interval, supplying an operative drive current to the firstLED element to produce illumination, wherein supplying the operativedrive current to the first LED element comprises adjusting a level ofthe operative drive current supplied to the first LED element based atleast in part on the corrected photocurrent.
 34. The method of claim 33,wherein the method further comprises comparing the correctedphotocurrent to an expected value; and wherein adjusting the level ofthe operative drive current supplied to the first LED element based atleast in part on the corrected photocurrent comprises adjusting thelevel of the operative drive current level supplied to the first LEDelement based at least in part on comparing the corrected photocurrentto the expected value.
 35. The method of claim 33, wherein the secondphotocurrent is measured before the first photocurrent is measured. 36.The method of claim 33, further comprising: measuring a firstphotocurrent induced in the least one photodetector while the respectivedrive current of the second LED element is supplied at an operativedrive current level to produce illumination and the respective drivecurrent of the first LED element is supplied at a non-operative drivecurrent level; and measuring a second photocurrent induced in the leastone photodetector while the respective drive current of each of thefirst LED element and the second LED element is supplied at anon-operative drive current level; subtracting the second photocurrentinduced in the at least one photodetector while the respective drivecurrent of each of the first LED element and the second LED element issupplied at a non-operative drive current level from the firstphotocurrent induced in the at least one photodetector while therespective drive current of the second LED element is supplied at anoperative drive current level to produce illumination and the respectivedrive current of the first LED element is supplied at a non-operativedrive current level to obtain another corrected photocurrent; andsupplying an operative drive current to the second LED element toproduce illumination, wherein supplying the operative drive current tothe second LED element comprises adjusting a level of the operativedrive current supplied to the second LED element based at least in parton the another corrected photocurrent.
 37. An illumination devicecomprising: first and second light emitting diode (LED) elements; and acontrol circuit operably coupled to the first and the second LEDelements, wherein the control circuit is adapted to: supply a respectivedrive current at an operative drive current level to each of the firstLED element and the second LED element to produce illumination; during ameasurement interval: measure a first photocurrent induced in at leastone photodetector while the respective drive current of the first LEDelement is supplied at an operative drive current level to produceillumination and the respective drive current of the second LED elementis supplied at a non-operative drive current level; and measure a secondphotocurrent induced in the at least one photodetector while therespective drive current of each of the first LED element and the secondLED element is supplied at a non-operative drive current level; subtractthe second photocurrent from the first photocurrent to obtain acorrected photocurrent; and at an end of the measurement interval,supply an operative drive current to the first LED element to produceillumination, wherein to supply the operative drive current to the firstLED element comprises to adjust a level of the operative drive currentsupplied to the first LED element based at least in part on thecorrected photocurrent.
 38. The illumination device of claim 37, whereinthe control circuit is further adapted to compare the correctedphotocurrent to an expected value; and wherein to adjust the level ofthe operative drive current supplied to the first LED element based atleast in part on the corrected photocurrent comprises to adjust thelevel of the operative drive current supplied to the first LED elementbased at least in part on comparing the corrected photocurrent to theexpected value.
 39. The illumination device of claim 37, wherein thecontrol circuit is further adapted to measure the second photocurrentbefore measuring the first photocurrent.
 40. The illumination device ofclaim 37, wherein the control circuit is further adapted to: measure afirst photocurrent induced in the at least one photodetector while therespective drive current of the second LED element is supplied at anoperative drive current level to produce illumination and the respectivedrive current of the first LED element is supplied at a non-operativedrive current level; and measure a second photocurrent induced in the atleast one photodetector while the respective drive current of each ofthe first LED element and the second LED element is supplied at anon-operative drive current level; subtract the second photocurrentinduced in the at least one photodetector while the respective drivecurrent of each of the first LED element and the second LED element issupplied at a non-operative drive current level from the firstphotocurrent induced in the at least one photodetector while therespective drive current of the second LED element is supplied at anoperative drive current level to produce illumination and the respectivedrive current of the first LED element is supplied at a non-operativedrive current level to obtain another corrected photocurrent; and supplyan operative drive current to the second LED element to produceillumination, wherein to supply the operative drive current to thesecond LED element comprises to adjust a level of the operative drivecurrent supplied to the second LED element based at least in part on theanother corrected photocurrent.
 41. The method of claim 33, furthercomprising: measuring a first photocurrent induced in the at least onephotodetector while the respective drive current of the second LEDelement is supplied at an operative drive current level to produceillumination and the respective drive current of the first LED elementis supplied at a non-operative drive current level; and measuring asecond photocurrent induced in the at least one photodetector while therespective drive current of each of the first LED element and the secondLED element is supplied at a non-operative drive current level;subtracting the second photocurrent induced in the at least onephotodetector while the respective drive current of each of the firstLED element and the second LED element is supplied at a non-operativedrive current level from the first photocurrent induced in the at leastone photodetector while the respective drive current of the second LEDelement is supplied at an operative drive current level to produceillumination and the respective drive current of the first LED elementis supplied at a non-operative drive current level to obtain anothercorrected photocurrent; and determining whether the another correctedphotocurrent is within a range based on a target value.
 42. The methodof claim 41, further comprising: adjusting a level of the operativedrive current supplied to the second LED element based at least in parton the another corrected photocurrent when the another correctedphotocurrent is determined to be within the range of the target value.43. The method of claim 41, further comprising: not adjusting a level ofthe operative drive current supplied to the second LED element based onthe another corrected photocurrent when the another correctedphotocurrent is determined not to be within the range of the targetvalue.
 44. The illumination device of claim 37, wherein the controlcircuit is further adapted to: measure a first photocurrent induced inthe at least one photodetector while the respective drive current of thesecond LED element is supplied at an operative drive current level toproduce illumination and the respective drive current of the first LEDelement is supplied at a non-operative drive current level; and measurea second photocurrent induced in the at least one photodetector whilethe respective drive current of each of the first LED element and thesecond LED element is supplied at a non-operative drive current level;subtract the second photocurrent induced in the at least onephotodetector while the respective drive current of each of the firstLED element and the second LED element is supplied at a non-operativedrive current level from the first photocurrent induced in the at leastone photodetector while the respective drive current of the second LEDelement is supplied at an operative drive current level to produceillumination and the respective drive current of the first LED elementis supplied at a non-operative drive current level to obtain anothercorrected photocurrent; and determine whether the another correctedphotocurrent is within a range based on a target value.
 45. Theillumination device of claim 41, wherein the control circuit is furtheradapted to: adjust a level of the operative drive current supplied tothe second LED element based at least in part on the another correctedphotocurrent when the another corrected photocurrent is determined to bewithin the range of the target value.
 46. The illumination device ofclaim 41, wherein the control circuit is further adapted to: not adjusta level of the operative drive current supplied to the second LEDelement based on the another corrected photocurrent when the anothercorrected photocurrent is determined not to be within the range of thetarget value.